Add pathlib support to input and output - #449

This looks good to me! But could you also make a few small changes so that this works in other routines as well?

• shclasses.shgeoid.to_netcdf() - we just need to update the docstring so that people know you can also use a Path.
• shio.shwrite(), read_igrf(), read_icgem(), read_dov(), write_dov(), read bshc(), write_bshc() - just copy to path to string conversion at the begging of each method.

And...

I noticed theshcoeffs.to_file() is a bit odd when using .gz files. This calls the shio method, which can deal with .gz files fine. However, shcoeffs.to_file() writes a text file by calling shio.write_xxx and then gzips it afterwards. I think that we can just remove all the gz and filebase stuff in shcoeffs.to_file() and just pass the Path directly to the dedicated write routine. If you don't want to make so many changes, I can easily do this myself later.

I did a new commit with the small changes you suggested:

• updating the docstring of shclasses.shgeoid.to_netcdf()
• adding the path to str conversion and associated docstring changes to the methods: read_bshc(), write_bshc(), read_dov, write_dov, read_icgem_gfc(), write_icgem_gfc(), read_igrf(),shio.shwrite()