Time semantics checking in hybrid.py
Added overwrite time data function
edit docstrings to reflect changes
Made _check_time_data consistent with docstrings
document overwrite_time in PwaSysDyn, HybridSysDyn
made timestep docstrings clearer
Replace 'continuous' with 'sampled'
replaced '&&' with 'and'
Unit tests for time semantics
Updated developer example to conform to changes
EXAMPLE: Delete pwa_timedata.py given lack of sig diff from pwa.py
TEST: Refactor hybrid_test.py
Merge branch 'master' into switched

Conflicts:
	tulip/hybrid.py
TEST: HybridSysDyn renamed to SwitchedSysDyn

to be consistent with refactoring of commit 5e227adff9559b14f3463ac1307eb052a38fdfd3
TEST: Correct hybrid fixtures to have consistent B, Uset dimensions