Allow `GPS` class to work with other image projections

Description of proposed changes

Added two general purpose functions useful for interacting with projections other than EPSG:4326:
- reproject convert X/Y coordinates between different EPSGs. It's more general than some of the other UTM conversion functions- for instance, to_latlon can now just call the reproject function to do the conversion.
- get_image_rowcol to find the pixel location nearest to some lat/lon location
Both functions are used in the GPS class to convert the gps station locations in lat/lon to whatever the image projection is, stored in the EPSG attribute
- Future note- I've also used this in a change to tropo_pyaps.py to run that script on non-EPSG:4326 images. That will be submitted after

Reminders

[ ] Partially addresses https://github.com/insarlab/MintPy/discussions/1010
[ ] Pass Pre-commit check (green)
[ ] Pass Codacy code review (green)
[ ] Pass Circle CI test (green)
[ ] Make sure that your code follows our style. Use the other functions/files as a basis.
[ ] If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.
[ ] If adding new functionality, add a detailed description to the documentation and/or an example.

insarlab / MintPy