Make CLIMADA purely `pip` installable

[peanutfun]

Is your feature request related to a problem? Please describe.
Make it possible to install a working CLIMADA installation with pip install alone. It's okay if some features do not work then. But CLIMADA in general should be able to be imported (top-level module)

Describe the solution you'd like

$ pip install climada
$ python
>>> import climada
>>> from climada.hazard import Hazard
>>> from climada.hazard import TCTracks
RuntimeError("cfgrib is not installed!")

Describe alternatives you've considered
Full installation is possible with Conda. cfgrib and eccodes cannot be installed with pip.

Additional context