Support for PathLike objects

[VeckoTheGecko]

Running the following:

from pathlib import Path
PLOT_FOLDER = Path("visualisations_particles")
...
pset.show(savefile=PLOT_FOLDER / 'particles', ...)

I get the following error.

     98     plt.savefig(savefile)
---> 99     logger.info('Plot saved to ' + savefile + '.png')
    100     plt.close()

TypeError: can only concatenate str (not "WindowsPath") to str

There are many ways of specifying paths in Python (strings, Path from pathlib and others). I feel that in general supporting these different ways of specifying paths would make parcels more flexible to users and their workflows.

This would be a more "quality of life" change, than a full blown feature. Regardless (as a fan of pathlib) I think this addition would be valuable, and would also bring parcels more in line with other libraries (eg. matplotlib) which support these other ways of specifying paths.

This also ties in with idea #1214, as having type annotations would make it clear the support for different parameter types (ie. that savefile only supports strings).

[erikvansebille]

Good suggestion, @VeckoTheGecko! I agree that we should support more pythonic workflows with path lib etc.

Would changing the info-message to an f-string as done in PR #1222 fix this? Can you check?