Penman-Monteith forcing (and the struggles of ESMValTool)

In PR #393 I attempted to add a generic forcing that would provide users with the meteo data required for the Penman-Monteith equation. I.e., air temperature, net radiation, vapor pressure deficit, wind speed.

However, I encountered a few issues:

- Debugging is impossible with esmvaltool/esmvalcore as it locks up the threads. Which makes fixing problems in the code take a very long time as emsvaltool is slow.
- The CMIP data is limited in the different variables that are available
- The CMORized ERA5 data variables do not align well with the CMIP6 variables
- wind speed consists of two components instead of mean horizontal speed
- radiation has the net long/short wave and downwards long/short wave, instead of the 4 up/down long/short components
- the dewpoint temperature is on a different mip table than all other variables. Which causes it to have different time bounds. Even though in the original ERA5 data from the CDS all data is on the same coordinates. Also, what is the difference between `Eday` and `day` because this is impossible to find in ESMValTool documentation, CMOR documentation, [CMIP documentation](https://github.com/WCRP-CMIP/CMIP6_CVs/blob/main/CMIP6_frequency.json).

Due to the issues piling up, we decided to put this on halt and finish up the PR without PM.

Here is how far I got:
https://gist.github.com/BSchilperoort/254fb3c5a54155b72e08861f40cb0e14

Of course this is a frustrating experience for myself, but I think this also touches on a more general problem: ESMValTool is built for evaluating climate model output. We're trying to use it for hydrological model input. This will clash in many ways, especially with ERA5 not fitting in CMIP properly.

While I do think that there is no alternative for ESMValTool when trying to get climate model data forcing data, I think that it might be better to give up on ERA5 + ESMValTool as it makes things a lot more complicated than it needs to be.