Dirty workaround for some string actions on pathlib obj.

[briochh]

Quickfix for issue with version 2 write of pest control file in PstFrom build_pst().

The issue relates to a few str actions that are undertaken with the model path now failing when that path is a pathlib.Path object. There is probs a more designed solution for this but I am just calling os.path.join() over the Path object to convert back to string. Unsure if this will cause issues, perhaps @aleaf has some thoughts on this?

[aleaf]

Hey @briochh - any Path instance can be converted to a string by wrapping it with the str() built-in (e.g. str(new_filename). I actually only started using pathlib a few months ago and was a little skeptical as to its value at first, but am now convinced it is the way to go. Not that everything can't be done with os.path (and some cumbersome platform wrangling when needed), but pathlib just works, usually with more minimal, readable code.

Anyways, in figuring it out, I found the pathlib doc to be pretty helpful. For the most part, Path instances are treated the same as string-based paths in python. It would be nice to implement this in pyemu as well. I'd be happy to help with this, either ad-hoc through pull request as things come up, or more systematically if we can come up with an inventory of all the places that would need refactoring.