General

• I ensured all of our classes have the "Plc" prefix, as I did encounter situations in which very generic type names had collisions and I had to use the fully qualified class name (including the package name). The "Plc" prefix reduces this risk.
• This model is now simplified as it currently doesn't support reading of multi-values. There are multiple options to add Multi-Values, but I haven't decided on which option I like best:
  • Rename PlcRequest/ResponseItem to PlcSingleValueRequest/ResponseItem and add a Multi-Value counterpart, wich handles the multi-value.
  • Add a PlcMultiValueRequest/ResponseItem, which simply references multiple PlcRequest/ResponseItem objects and provides a "get(index)" method.
  • Add a getNumValues() (Not insisting on this name) method and give each of the accessors an "index" property (eventually additionally a no-args version which defaults to index 0) (I think I prefer this one but didn't want to blow up the class-diagram too much).

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PlcField (Aka Address)

• I Renamed Address to PlcQueryPlcField
• I still think there is no immediate need to define any fields or methods in this interface as I haven't seen any common ground yet. Even if we are currently adding "TYPE" information to most queries, in Modbus the type is directly tied to the address you are reading. So If you are reading a "Register" or a "Coil", the datatype is fixed.

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• I removed the "getLister" method as I haven't seen any protocol or use-case where this actually made sense.

• I changed the "parseAddress" to "prepareQuery" as it sort of correlates to the "prepareStatement" of JDBC ... and some prepareField". This abstract method has to be implemented by every Driver and it handles parsing of the field description for each protocol. Some times I could imagine, that more work is involved than simple parsing of a string (For Beckhoff ADS and EtherNet/IP this method could actively connect to the corresponding resource so in future read operations only the connection-id has to be provided)
• I added a "prepareQuery" field which sort of correlates to the "prepareStatement" method used in JDBC. It is used to parse a query string and produce a corresponding PlcRequest object. 
  • A query like this: "SELECT '%DB8.DBX3:INT' AS parameterX, '%DB9.DBW4:DOUBLE' AS parameterY, '%DB2.DBW2:BYTE[10]' AS others" would return a PlcReadRequest Object with initialized PlcReadRequestItems. 
  • A query like this: "SELECT '%DB8.DBX3:INT' AS parameterX, '%DB9.DBW4:DOUBLE' AS parameterY, '%DB2.DBW2:BYTE[10]' AS others WITH INTERVAL 1000" would return a PlcSubscriptionRequest Object with initialized PlcSubscriptionRequestItems. 
  • A query like this: "UPDATE '%DB8.DBX3:INT' = :parameterX, '%DB9.DBW4:DOUBLE' = :parameterY, '%DB2.DBW2:BYTE[10]' = :others" would return a PlcWriteRequest Object with initialized PlcWriteRequestItems. 
  • ... (Actual format should be discussed).
  • The parser for this would be implemented as Antlr4 Parser used in the base PlcConnection class and is inherited by the concrete driver implementations. The driver dependent part is the parsing of the field itself, but this could be handled by the abstract prepareField method (This should actually be quite easy for me to implement).

PlcRequest & PlcResponse

• I changed the List for referencing the items with a Map, so we can directly reference items.
• I removed the "single-item" convenience methods.
• I added getItem(name) methods for directly referencing a given item.
• I am not that happy with the WriteRequests: Currently you can create one ReadRequest instance and reuse it as often as you like, however when writing, a new object has to be created every time. 

PlcRequestItem & PlcResponseItem

• I removed the datatype both from the properties as well as the generic type as I think type information - if needed - should be inside the query objectfield object.
• A request item now has a name, which allows to access it via it's name.
• PlcReadRequestItem no longer has a "size", as that's part of the query field now. Some protocols ... such as Modbus (I think) ... don't support this concept.

• NOTE: I would suggest to not remove PlcReadRequestItem or PlcWriteResponseItem class entirely, even if we could simply make PlcRequestItem and PlcResponseItem non-abstract. For simplicity of the API I would opt for keeping it ... so we always have matching pairs of PlcXYZRequestItem and PlcXYZResponseItems.

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