Add MTG and MTG_pvlib configs

[KristianHMoller]

Adding configurations to allow running with MTG data produced by specMAGIC (https://github.com/dmidk/specMAGIC).

Specifically, two configurations are added: MTG and MTG_pvlib. The former uses clear sky calculated by specMAGIC directly, while the latter uses climatological clearsky calculated via the pvlib package. The latter is much slower, but does not require specMAGIC clearsky.

MTG_pvlib requires GHI values for the requested time as well as the three preceding time steps.
MTG additionally requires CSR (clear sky) values for the four same time steps as well as 24 time steps from the previous day starting at the requested valid time and going forward in time.

To make running with different datasets easier, the option of providing more configuration parameters to config.yaml has been added. This overwrites the hard-coded defaults for the parameters previously given as environment variables. Environment variables still take priority. For now, the value of 10 for input_data_frequency_minutes has been added for the MTG configurations directly.

To run with MTG data, one simply needs to export the SATELLITE_DATA_DIRECTORY to point to the out directory containing the specMAGIC output directories:

export SATELLITE_DATA_DIRECTORY=<PATH/TO/SPECMATIC>/out

sunflow --run_mode files --dataset MTG_pvlib --time <datetime in ISO8601 format>
sunflow --run_mode files --dataset MTG --time <datetime in ISO8601 format>

[ElloiseFangelLloyd]

Both cases work well for me!