replace pformat by new: hybrid.indent

because pformat uses the __repr__ whereas for some tulip classes only an (informal) __str__ is defined (after __repr__ where removed where they are not official representations).
new: multiproc_discretize_switched

using multiprocessing and its logging facilities.
This fully parallelizes mode partitioning and finding feasible transitions per mode on the merged partition.
Partition merging can be performed in LOGTIME on a linear number of processors in the number of partitions.
use matplotlib Agg backend in switched examples

to avoid X11 over ssh