[Feat] Improve JSON in v.to.db

[petrasovaa]

Is your feature request related to a problem? Please describe. There are couple things to address in the current JSON schema, mainly consistency.

• [ ] The key "unit", should be renamed to "units" and should look like totals:

    "units": {
        "length": "kilometers"
    },
    "totals": {
        "length": 38.839528111915129
    },

This will be more consistent and in the future if we decide to implement printing multiple geometry attributes, the units could be different for each attribute.

• [ ] Also, totals and units should appear everytime, but if it does not apply, it should be empty {}.

• [ ] update tests

• [ ] add example to documentation, a simple Python one showing the JSON output

[NishantBansal2003]

Has this already been addressed in https://github.com/OSGeo/grass/pull/4036, or is there something else that still needs updating?

[petrasovaa]

I somehow forgot this was implemented, I reviewed the implementation and changed the PR description.