MAINT: Slight refactor.

Significant impact on performance due to namedtuple class construction.

http://nbviewer.ipython.org/urls/gist.githubusercontent.com/cpelley/1269dc60c41944d9c504/raw/ab229432ba73b617744339222245334f26494a8d/regrid_performance.ipynb

Currently can cause up to %135 increase in the time it takes to regrid due to redundant namedtuple construction! (restricted to the simpler case where the array is not masked and mdtol=1).  For the case where we deal with masked arrays, nametuple construction accounts for ~23% of the total regrid!  Either way, this is an unnecessary redundancy.

I appreciate that I should really point this to v1.7.x however I would like to have the changes available from master available to me including this one.  Would it be ok for me to make a duplicate PR pointing to v1.7.x to keep everyone happy?