Add bilinear interpolation between rectilinear grids.
Includes review changes.
Merge pull request #442 from rhattersley/regrid-bilinear

Add bilinear interpolation between rectilinear grids.
Safely build PyKE rule base.
Merge pull request #435 from bjlittle/pyke_rule_compile

Safely build PyKE rule base.
Added cfchecker to travis-ci