Add support for H5MD files

[JFRudzinski]

Add new atomistic parser for H5MD file/schema

[JFRudzinski]

@ladinesa Here is the parser that I am working on for custom/general uploading of MD data.

The idea is to build from the H5MD schema---a very simple schema for organizing MD data within an hdf5 file proposed already several years ago.

This schema is insufficient for building a parser in NOMAD, but I have now extended it to be slightly more specific and slightly more compatible with NOMAD. I still need to write docs for this extended schema.

In the end, you can view this approach as half-way in between the traditional parsing of simulation output files, and directly writing your data into the NOMAD schema. It provides a more accessible way for users to upload custom calculations and parameters.

Probably it would be good to sit together and discuss this a bit more so I can explain the reasoning/strategy behind this.