Enable parametric input for CTLearn models

[TjarkMiener]

Nice approach by arXiv:1910.09435 which includes the hillas parameters in DL models.

• Enable (writing &) reading of DL1b parameters in DL1DH. (Done by @LucaRomanato @TjarkMiener)
• Modify CTLearn model to be able to include DL1b parameters whenever the user selects those in the config file at training phase.
• Implement MLP models for DL1b parameters only (w/o DL1a images).