Improved reference schema for model parameter validation

This merge requests implements an improved reference schema for the model parameter descriptions:. This follows now the widely used (industry standard?) json-schema, which allows to use public tools for the validation. This is necessary, as it is very hard to keep track of the allowed / required / optional options.

In detail, this merge request implements:

• replacement of reference_schema/model_parameters.yml by reference_schema/parameters-jsonschema.yml following json-schema
• new validation tool scripts/validate_parameter_descriptions.py using the python tool json-schema. This is super simple now! The old one was really a pain, but this is a simple 60 lines script.
• adaption of parameters/Photon_conversion.yml to allow to easily write a json schema (only change is that the parameters are now part of a list called parameterlist)

Only parameters/Photon_conversion.yml has been adapted yet, all other parameter files requires. The schema also will need a bit further refinements, especially the required lists).

Note that the base of the schema has been derived using https://codebeautify.org/yaml-to-json-schema-generator.

No detailed merge request is required, just browsing through. This will get refined with the upgrades of the other parameter lists.