Era5 implementation

[lthUniBonn]

Hey Mark,

this part of the code should adress the era5 implementation - reading all files is handles in the read_requested_data.py, configuration for this is set in the config.py file.
The height calculation utils for era5 are basically copied from the awe-era5 repo.

For now (for better comparison between era5 and dowa results I use the dowa lat/lon indices to define a position. In the long run I think we will want to define latitude/longitude and then read the data
As it was not clear how we wanted to handle multiple locations, I didn't want to code this before, just to avoid double work/concepting. When you give your input on multiple locations next week it will probably be much clearer.
Multiple locations / location settings in general could thus be addressed in a new issue?

in my repo, the evaluation branch includes the code to calculate the pc/clustering differences. I'll start a pull request for that a bit later, when the tests we discussed are also implemented.

Cheers,
Lavinia