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.

[JoyMonteiro]

I don't mind spending some time profiling different libraries. Which ones do you suggest?

Also, this library would need to be registered with sympl. Do you propose adding an API to
do this, or will it replace pint as a dependency?

[mcgibbon]

I'll need to write an internal backend API and wrappers for each library. This would technically make pint an optional dependency, but I'd say we should keep it as is to make sure there is at least one unit library installed, because it is pure Python with few dependencies of its own. We could default to cfunits when it is present.

[mcgibbon]

Let's move discussion over to an issue on Sympl.