bugfix: if no cont_props passed to discretize
new arg to discretize: plot_every

for plotting during partitioning every k iterations, to reduce the delay introduced by plotting, while maintaining a view of the algorithm's progress
Merge branch 'master' of https://github.com/tulip-control/tulip-control
bugfix: don't use same variable name for node and dict value
bugfix: initialize iter_count when no plotting"
rename examples
update png names in renamed examples
bugfix: plot discrete vector field
refactor HybridSysDyn and test it

env_labels, disc_sys_labels become explicit arguments to HybridSysDyn.__init__.

env_labels, disc_sys_labels are now properties, so protected from changing.

If no env_labels or disc_sys_labels are provided, then they default to ints, with warning. The ints are not saved, but instead returned using range upon access.

If the number of mode labels does not match disc_domain_size, or len() is undefined for them, then again the default ints are used.

The previous version defined labels as given, even if they didn't match the dynamics keys, in which case ints were compared to dynamics keys.

- logging messages
- docstr restructured
allow for missing mode combinations in HybridSysDyn
warn if missing env,sys mode combinations in HybridSysDyn
check each HybridSysDyn mode has PwaSysDyn dynamics
new: HybridSysDyn.__repr__
new: PwaSysDyn.__repr__
bugfix: check_convex when taking union in discretize

introduced in 0def035a65b0409865440449c97d463a3061f348
Merge branch 'master' into narciscott
DOC BUILD: Add hybrid.py, and change polytope to be package