This requires reference types that can be null to be annotated by a ? operator, similar to value types.
This gives the advantage that the compiler can warn against any null dereference exceptions, of which this commits elimits a few.
To make the underlying protocol implementation not any more complicated and to eliminate existing problems, and not that precise error reporting, I replaced some return null statements with explicit Exceptions. This lead to the assumption that those core protocoll functions always return non-null objects if they do not throw, making the PLC code simpler.
Adjust some NotConnected tests to look for explicit PlcException instead of NullReferenceException.
* Refactor PLCAddress
- Change fields into properties
- Apply PascalCase naming to all properties
- Make Parse public static with out parameters
* Support creating DataItem from string
* Rename PlcAddress.Address to StartByte
