MNT: change single letter internal variables

E.g.
* `c` --> `coordinate`
* `r` --> `result`

or something similar, that make sense in the actual context and not extreme abbreviation. This can be done one module at a time and is very much suitable for folks new to the package, to open source, or to python.

The issue names no files, tests, or entry points. Start by choosing one astroquery module and inventory its single-letter internal variables; use the surrounding context to select clearer names, then verify that the module’s existing behavior is unchanged.

Written by the indexing model from the issue text.