RTG SIMS depth profiles

[budschi]

This issue collects open tasks for the RTG SIMS schema plugin.

• add plots of the depth profiles
• extend schema to handle concentration profiles (calibrated measurements)
• clean up leftovers from NOMAD's example schema plugin
• add base classes

[budschi]

Basic functionality of the schema plugin is there. Next steps
/ Involve SIMS users and RTG
/ Add more base sections
/ Sample ID or reference to a sample
/ Substance in the matrix and depth profiles