FORCING classes have hardcoded path to forcing data

[tingeman]

With the recent code restructuring, FORCING classes no longer receive the forcing data path specified in the run file.
Instead, FORCING classes have hardcoded forcing paths.

In the new PPROVIDER class, all classes are instantiated the same, and therefore we cannot pass specific parameters to some instantiated classes.

Possible solutions:

• Pass all paths (result_path and forcing_path) to all instantiated classes, even those who don't need them
• Pass the PPROVIDER instance to new_class during initialization, and let the class itself grab additional parameters it might need from the PPROVIDER instance
• Require user to manually set the forcing_path after instantiation. In practice this would be handled by the TILE class right after retreiving the FORCING instance from the PPROVIDER

Which would be the better way?
Other suggestions?