Desalination model

[FranziPl]

This PR will add the second solar model to the repo, the desalination model. The structure is similar to the forst one, the solar cooling. The requirements are also valid for both.

[c-moeller]

I get an error, running the system:

Traceback (most recent call last):
  File "desalination_main.py", line 16, in <module>
    from desalination_precalculation import run_precalculation
ModuleNotFoundError: No module named 'desalination_precalculation'

[FranziPl]

I added the missing file, so the error shouldn't appear anymore. And I changed the code according to your suggestions.