regrid_time does not have promised functionality

[rebeccaherman1]

The stated goal of regrid_time is 'Align time axis for cubes so they can be subtracted', but it does not accomplish this goal. It takes two inputs: a cube, and a frequency. Since it does not accept two cubes, I imagine it's meant to simply remove evidence of time information that could conflict between cubes, specified by frequency.

I would like to subtract the value at one month from another. To accomplish this, I call extract_month using two different months. Then, I understand I should apply regrid_time to each cube, and I imagine the correct choice of frequency is yr, though it's not perfectly clear from the documentation. When I follow this procedure, I am still unable to subtract one from the other, getting the error Coordinate 'month_number' has different points for the LHS cube 'surface_temperature' and RHS cube 'surface_temperature'.

Examining one cube at a time, I see that using extract_month creates an Auxiliary coordinate called month_number. No matter what frequency I use, regrid_time adds additional coordinates day_of_month and day_of_year rather than removing auxiliary coordinate month_number.

@valeriupredoi? 🙃

[bouweandela]

Indeed there are opportunities for improving this preprocessor, so if you have time to work on this your contributions would be most welcome. See #2106 and #744 for related ideas.