cc_location/cc_elevation assumptions

[mdsumner]

These need to be generalized out:

• cc_location returns RGB for mapbox images and DEM, but single-band raster for ""elevation-tiles-prod"
• cc_elevation returns single layer for mapbox DEM only

Modify so

• cc_elevation defaults to "elevation-tiles-prod"
• OR, probably better write some new functions with sensible names and include types that are RGB or raster, not either

[mdsumner]

might be better to move to terra once that's out

[mdsumner]

removed the @2x distinction for terrain, so all tiles are 256x256 by default (so identical calls give the same grid for the matrix party)

also see #32