[hiopy] Support arbitrary grids

Motivation

We want to support other grids than healpix (e.g. Meteogram / native ICON grid)

Task description

• extend the grid_id and def_grid methods such that it also looks for coordinates: clon clat attribute, which contains the variables names of the points that are used as a grid.
• Documentation
• Add a test to the CI

The extension for grid with cells if considered later.