Issues with ERA5 specific humidity and surface pressure computation

[navidcy]

At https://regional-mom6.readthedocs.io/en/latest/demo_notebooks/reanalysis-forced.html#Step-7:-Set-up-ERA5-forcing:
there are some issues:

1. You should also say you are using the Antoine equation for vapour pressure as a function of temperature.

2. q = 0.622 * RH * P_sat / P = 0.622 * P_vap / P

3. Where does the 0.001 come from?

4. The T in the first equation should really be T_d.

found by @CallumJShakespeare

[navidcy]

We need to clear up this code here

regional-mom6/regional_mom6/regional_mom6.py

Lines 1612 to 1619 in 305f169

	## Calculate specific humidity from dewpoint temperature
	dewpoint = 8.07131 - 1730.63 / (
	233.426 + rawdata["2d"]["d2m"] - 273.15
	)
	humidity = (
	(0.622 / rawdata["sp"]["sp"]) * (10**dewpoint) * 101325 / 760
	)
	q = xr.Dataset(data_vars={"q": humidity})

It might be cleaner if we convert this into a function.
But definitely we should avoid hardcoding numbers in, but rather define variables with values so that it's obvious what's happening here.