Update mods docstring

[yumengch]

This pull request relates to the issue #23.

I don't see there are general documentation in L96/63 modules. Of course, I could be wrong, but I added some words for the mods/__init__.py.

Remaining with issue #23:

• It is doable to move dstep_dx and d2x_dtdx to an extra.py module.

  However, to make it clear, I believe it means, in LUV models, a new class of model_instance_extra should be used, which should (not?) be an inheritance of model_instance. We need to ensure the model parameters communicate with these modules.

• It was a hectic week for me so I can only do little for DAPPER.

[patnr]

I added some more notes as well.

I think putting the TLM/Adjoint stuff into an extras file is only relevant for Lorenz63 and Lorenz96, since they are the "beginner" models. I am doing this now.

No worries! And thank you!