Add Type Hints to all classes, methods and functions.

[jgd10]

This is a pretty big task, but I think it would be worthwhile to have type hints wherever it is sensible. We already have them in a few places. E.g. in mesh_commands.py

class _MapdlMeshingCommands:

    def n(self, node="", x="", y="", z="", thxy="", thyz="", thzx="",
          **kwargs) -> int:

But ideally type hints should be present on every callable. I think it's a fairly uncontroversial addition, and I am happy to go through and start making these changes, although I don't suspect it'll be done particularly soon!