[FEAT] make 3D momentum output files.

[AtheerAmmar]

make an output file type that has all of the momentum source terms, please!

[abukowski21]

This was added in #84.

Leaving the issue open since we forgot to add the output types to the documentation

• See the documentation webpage
• The file to edit is outputs.md

The rest of the outputs need verification (and may need tidying) so I'll self-assign this issue & can help on it