Recipe for PEL upward continuation

The recipe outlines how the PEL should be used.
gravmag.sphere was missing dens kwarg

All of the gradient components were missing and all missed the docstring
entry.
Added test in EQLGravity for joint inversion

Using gz and gzz together to estimate a layer. This tests functionality
of MultiObjective with multiple data types.
Merge branch 'inversion' into no-multiobj
doctest for joint inversion

Added a doctest for joint inversion in fatiando.inversion
Merge branch 'inversion' into no-multiobj
Separated the results of fit in 2 attributes

The first is p_, which contains the parameter vector. Next, is
estimate_, which contains a possibly formatted version of the estimate.
This way, I can overwrite fit when subclassing to transform the
parameter vector into something more useful (like something from
mesher).
Created base classes for eqlayer

The PEL base overwrites fit to convert the coefficients to physical
properties.