[core][process] main rework of the processes.

- now process can take an output and an output_dict for storing the result in a multiprocessing.Proxy object
- rework process action to avoid several call to pymca `processSpectrum` function
- add an option to XasObject `to_dict` function to simply store raw data (which is the reference), but also to keep processing results to be used later.
This is need for the 'non gui' workflow processing which uses a dictionnary instead of a XasObject.
3 jobs for fix_10 in 4 minutes and 18 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed test:python3.5-stretch-pyqt5 #41741

00:04:11

passed test:python3.5-stretch-pyside2 #41742

00:04:10

passed test:test-xas-tutorials #41743

00:04:18