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