Make Trigger.crossing_time_expr more robust #37
Description
The method crossing_time_expr in the Trigger class is used to return an expression in terms of t (time) when the trigger threshold is crossed. Currently it can simple Trigger expressions consisting of a single inequality, or multiple inequality clauses combined by sympy.Or.
However, crossing-time expressions for trigger expressions including sympy.And should be able to be handled using piece-wise expressions (I think). Will require some careful thought though.