Add `shp_to_exp()` to convert shapefiles to exp files.

[lawrenceabird]

Most ISSM functions rely on *.exp files. Most data are stored/shared using *.shp files. This function should convert *.shp files to *.exp files. This functionality exists in the legacy python implementation, but needs to be added to pyISSM.

[lawrenceabird]

We should probably add the equivalent exp_to_shp() function as well, for completeness.

[lawrenceabird]

This is addressed in #192