FIX: safety PLCs are located in _Config/SPLC

[klauer]

Context

Closes #288

@ghalym discovered this which I summarized in #288

Background

Project is an overloaded tag in TwinCAT XML files. It can be:

• A TcSmProject
  • The .tsproj root node
• A top-level project -> TopLevelProject
  • This contains PLCs, motion, safety, IO
  • Found in .tsproj under TcSmProject
• An embedded PLC project -> Plc
  • Found in .tsproj
  • Project inside a Plc node underneath a TopLevelProject
• An external PLC project reference -> Plc
  • Marked with "File" or "PrjFilePath" attributes
  • Found under TcSmProject / TopLevelProject / Plc
• The top-level of an external .plcproj project -> PlcProject
  • Found in the .plcproj file referenced via TcSmProject / TopLevelProject / Plc
• A safety PLC (found under a Safety node) -> SafetyPlc
  • Found under TcSmProject / TopLevelProject / Safety
• A give-up fallback Project container that is pretty much ignored in pytmc

Changes

This PR adds support for the last option.

[klauer]

@ghalym reports that this fixed her issue. Unless I can find another project with a Safety PLC in it to test with, I think this is good enough for now.