.tim to .bc converter

[arthurvd]

Is your feature request related to a problem? Please describe.
As part of #621, we can convert boundaries in an old ext file to boundaries in the new ext file, but this currently only concerns the ext files, not the underlying forcing files.
Under this issue we also want in the old input the old .tim / .cmp / .t3D files to be automatically converted into newly created .bc file.

Describe the solution you'd like
For every boundary quantity in the ExtOldModel.forcing list, check for the filename attribute whether it's a PolyFile and if so, then find whether there exists:

1. either a .tim file (with the same stem as the polyline filename)
2. or, alternatively, a .cmp file (same stem again)
3. or, as last check, a .t3D file (same stem again)

Additional context
The above conversion to some extent includes specific rules for boundary data. But, partly the .tim to .bc converter can also partially be based on generic .tim to .bc conversion: if you pass quantity name, function type, time interpolation settings and maybe some more settings to a utility function, then then creation of the ForcingModel object can furthermore be implemented generically.