Constants and their units for cube calculations about esmvaltool

[mattiarighi]

Good idea. Some constants are already collected in
https://github.com/ESMValGroup/ESMValTool/blob/master/interface_scripts/constants.ncl
but for NCL only. The constants listed there should be quite common and also available via scipy.constants, so probably there is no need for further definitions.

@ESMValGroup/esmvaltool-coreteam

[mattiarighi]

Is this still relevant? @jvegasbsc

[jvegreg]

I don't think we need to create a wrapper around this for python:

1. Units: iris is doing the unit computations when using cubes and we can add this computation manually using cf_units when needed

2. Constants: It's nice that we agree in one source of constants to be used in the whole applications, but we can use scipy.constants directly and contribute anything that we need that is not there.

In order to be consistent with ncl constants file, I would recreate it when needed using the values obtained from scipy