Draft: Base section for crystal structure file parsing into the metainfo

A nomad User asked about how he could deposit crystal structure files from a structure generative ML model into nomad. Whilst the general discussion about how we are going to design a workflow for ML-generated data is gonna take some time, I have created a base class to parse structure files into the nomad metainfo. At the moment, it writes the run.system section, but I understand from the discussion in #1038 (closed) that the section run is reserved for DFT codes. Should it be ok to put a System section in data and the parse this to Results?

@mscheidg @himanel1, any advice on how would you approach this?

Thanks!