MAINT: minor performance improvement

Merge two loops and move invariant of switch case out of inner loop.
Improves performance by 5% - 10% depending on the order.
MAINT: tell compiler that edge interpolation is less likely

This should be true for most cases where performance is relevant.
Improves performance by about 5% on images > (500, 500) with order three.
MAINT: add map_coordinate tests with non-continous arrays
Merge pull request #503 from juliantaylor/ni_interpolation-performance

ENH: Ni interpolation performance improvements.