Title of eqlayer upcontinue was missing

Sphinx didn't find it because there was a newline at the top of
the file
Docstrings and tests for regularization

Added docstrings for classes in inversion.regularization with
doctests. Improved docstrings in inversion.base as well
Docstring of fatiando mentions new inversion pkg

Updated the information in the docstring of the fatiando package to
include the new inversion framework.