Clarify use and data types in docs

[mdtanker]

Description of the desired feature:

To help users understand what this package is for and whether it will be helpful to them, we need to add more info to the documentation about what type of research this package is intented for (geospatial generally, but currently focused on geophysics / geology / ice dynamics since that is what I (the main contributor) happen to research.

It should be clear that other geospatial data types outside of my field (biology, climate, etc) are welcome to be included, but at the request of users.

Along these same lines, we should clarify what datatypes the package is set up for. Mostly gridded datasets in projected coordinate systems (EPSG3031 for Antarctica, or EPSG3995 for the Arctic, or EPSG5938 for Greenland). Minimal plotting support for spatially discrete (point) data. No plotting support for geographic coordinate systems.