88 add xrf measurents

[fabianschoeppach]

Still work in progress. @hampusnasstrom wanted to see a draft. 🙂

[fabianschoeppach]

Overview on the work which has been done:

• Created a schema with XRayFluorescence etc. closely mirrored the structure from xrd schema
• a ELNXRayFluorescence reference implementation which should be extendable to handle other file formats
• specific reader for extracting data (multiple layers, elements, elemental lines and intensities) from the UBIK XRF file format
• Include XRF test files

Things which still are unclear/not implemented

• Sample handling: Up to now only Sample id as string
• UBIK files do not contain information regarding measurement setup
  (energy, spot size, integration time etc -> XRFSettings)
• Library measurement as HZB stuff not included