Test reverse monotonic coordinate interpolation

[mpu-creare]

Description
This line https://github.com/creare-com/podpac/pull/479/files#diff-1a9863f052004c4aa6103bbbbb2294d0862d15aac0ec91a5becaff0ca9d43097R120

was 'fixed'. It's really a bug in xarray, but with our usage I'm not sure setting this to False always gives the correct answer.

Need to add a test and then do the right thing -- if needed.