r.object.thickness: update after csv class refactoring

[pzatelli]

Update module after csv class refactoring.

[echoix]

Hmm, why is the target branch master, and so much failing checks?

[petrasovaa]

Right, I just reviewed the change itself.

[echoix]

I'll try changing the target base branch, let's see if it messes everything up or not.

[echoix]

Didn't work, it should be a cherry-pick, or recreated starting from the correct branch (grass8).