New inference

[jcblemai]

Functionalities to support several inference algorithms, make flepimop install a one-line thing, overhaul file saving and analysis

• convert the internal representation of epi states from data frame to xarray
• save epi state only on command
• update documentation for new packages
• implements class for statistics
• implement likelihood computation
• implement emcee as a script
• rewrite resume and continuation batch
• rewrite current inference algorithm
• make it possible to batch with and without MPI, or on single node.