Unit handling code is slow

[mcgibbon]

@JoyMonteiro brought up that currently when run in SCM mode, unit conversion checking takes up a significant amount of CliMT's runtime. We may want to add a faster backend for unit handling.

See climt/issues/75 for original discussion of this issue.

cfunits is a good candidate as a backend, but it appears to only work on Python 2. When I run cfunits-1.5.1 on Python 3, I get:

ValueError: Bad python version: cf requires 2.6 <= python < 3.0. Got 3.5.2

The more official package appears to be cf-units, hosted by the Met office, but that is also incompatible with Python 3 according to their docs and my attempt to download it through conda. We could try to fix it to work with Python 3 and submit a PR to them.

Or we could look for another option.

[JoyMonteiro]

cf-units now works on Python 3.6/3.7. Can we restart discussion/work around this issue?

[JoyMonteiro]

I was playing around with the code, and if I change this line:

sympl/sympl/_core/units.py

Line 81 in f97e211

elif unit_registry(value.attrs['units']) != unit_registry(units):

With an equivalent statement using cf-units, the code is twice as fast. cf-units supports all
aliases setup in this file. It does not understand 'dimensionless', and that needs to be handled. Otherwise, this should be easy to optimize -- use cfunits if it exists.