Rough beginnings of the variogram class.
Merge remote-tracking branch 'geotrellis/master' into kriging
Laid out structure for Ordinary Krigin Interpolation (with linear model for semiovariogram).
Refactors to Variogram.
Merge pull request #1 from lossyrob/kriging

Refactors to Variogram.
Complete Variogram, start of test class.
Updated tests.
Fixed (-) function in Point.
Variogram complete (uncommented and littered with print statements)
VariogramSpec complete (w/ extra linear regression tests)
Renamed to semivariogram.
Added comments.
Removed regression tests.
New lines at the end of the files.
Gave the ability to have zonal summary use cached results.

- Added a .cached method to DataSourceLike that caches the results
  of the elements on the local machine.
- Refactored zonal summary methods to take in cached results
  to be used on the full tile intersections.
add double support to max focal operation
add double support to min focal operation
add double support to standard deviation focal operation
add double support to sum focal operation
Merge remote-tracking branch 'mark/FocalDouble'
Some fixes and tests.
Merge pull request #671 from Markko/FocalDouble

add missing double support to Focal operations issue: 630