[refactoring] split types.py class

Description / Goal

TODO list

• add Spectra class with API
• split types into a submodule
• clean API / call from xas_object
• add offset information -> must be done in another PR. There is some GUI to be updated too.
• doc
• add test
  • add test on the spectra API (using python slices...)

INFO (how to use...)

close #24 (closed)

Any warning ?

associated materials (screenshot, test scripts...)