Rename L2Norm to Misfit and moved it to base

L2Norm is not usable by itself, so it is a base class.
Renamed it to Misfit to be more intuitive (and since I won't be implementing
an L1 norm anytime soon and it can be substituted by a re-weighted L2 norm
solution).
Filled docstrings for Misfit
Filled missing docstring in gravmag.euler

The docstring of the covariance method was missing.