Clean-up ad-hoc xgcm.Grid.cumsum(..., reverse=True) operations

[hdrake]

Most of the lines of code in /xwmb/budget.py are currently dedicated to performing reversed cumulative sums. We can already probably clean this up quite a bit by making a utility function to perform this operation, rather than duplicating the code several times throughout the WaterMassBudget methods.

If xgcm.Grid axes become directionally-aware (see xgcm/xgcm#337), then it should be straight-forward to allow reversing the direction of xgcm.Grid.cumsum or xgcm.Grid.cumint (see xgcm/xgcm#609).