Wrap all docstrings to 79 characters

[santisoler]

Description of the desired feature

After fatiando/verde#177 raised by @prisae, fatiando/community#9 and fatiando/community#10 it has been decided that all docstrings must be wrapped to 79 characters per line.
We should warp all the existing docstrings in Rockhound to 79 characters and also configure flake8 to check for this when running the style checks.

Sadly, there's no way (yet) to automatically change all docstrings, but at least we can use flake8 to raise lines that fail. This can be done by setting max-doc-length to 79 characters on the flake8 configuration under setup.cfg.