Evaluation algorithms separated from misc

Created a new nipype.algorithms.eval module for methods performing
error, distance or similitude computations over images.

I think this is better than having them mixed up with, e.g.,
nifti or csv files management.

I also add a new interface: ErrorMap, that computes a distance
function between two images (e.g. two displacement fields). Several
metrics will be available, however now I contribute only with
squared differences maps.