non-MPI version for (new) Python interface (only affects new_python)

Always compile a non-MPI version of the (new) python library, which can be loaded by setting the GENERICIO_NO_MPI env variable. Also add the PrintStats boolean to the python interface.

Background: on some systems, MPI is not supported on the login node (cooley) or on jupyterhub instances (summit), and the "new" python module cannot be loaded on this system. This commit compiles two python modules (with and without MPI, or only without MPI if MPI is not available during compilation).

By default, if available, the MPI version is loaded. The non-MPI version can be forced to be loaded as follows:

import os
os.environ['GENERICIO_NO_MPI'] = 'True'
import pygio

The writing of GenericIO files in the python module is not supported if MPI is disabled.