[KristianHMoller]

This PR adds the option of providing a custom output filename format as opposed to this being hardcoded. The custom output filename format can be specified for a given dataset in config.yaml as currently done for KNMI_MTG.

Additionally, the default output filename format has been changed to have an ISO 8601-compliant datetime format.
Default output filename format: SolarNowcast_YYYY-MM-DDTHHMMZ.nc
Selected output filename format for KNMI_MTG: SolarNowcast_MTG-CPP_YYYY-MM-DDTHHMMZ.nc
For legacy reasons, the default remains unchanged with KNMI_MSG: SolarNowcast_YYYYMMDDHHMM.nc

With this PR, the filename_format previously used in the config.yaml for the input filename format has been changed to input_filename_format, so runs with old config.yaml files will fail.

Additional flexibility has been added to allow the INPUT_FILENAME_FORMAT and OUTPUT_FILENAME_FORMAT environment variables to overwrite the corresponding filename formats so that the formats can be set without changing the code.

Finally, the dataset variable has been added to the metadata of the output NetCDF file with the label satellite_data_source for transparency.

[elenibriola]

Thanks for the changes, Kristian! The implementation looks good overall and I like the additional flexibility for configuring filenames through environment variables.

I have a few comments/questions:

1. Would it make sense to include the dataset name in the default output filename to avoid potential collisions when different datasets are processed for the same timestamp and stored in the same location?
2. I will add a few unit tests for the filename generation and configuration precedence. I will start the pr for it these days. I think it is time.

Otherwise the changes look good to me!

[KristianHMoller]

Excellent idea with the dataset in the default output filename.
Implemented with:
c3c4a0e

[elenibriola]

I have nothing else to add. Looks good to me! Nice work!