Removed MultiObjective

Replaced it by over-writing the methods in Objective it self to sum
another function and multiply by scalar. The advantage is that the
result of the sum A + B is a copy of A with wrapped hessian, gradient
and value methods. It still retains all its properties (like overwritten
fit method, custom methods, etc).
Keep in mind that A + B + C will be a copy of A.

Still have to overwrite predicted and residuals in Misfit to return a
list of predicted and misfits, like MultiObjective did.
Merge branch 'inversion' into no-multiobj
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.
Finished PEL

Finished the implementation, doctests and cookbook recipes.