Make Trigger.crossing_time_expr more robust

[tclose]

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.

[yush-1018]

@tclose,
Hi! I'm interested in tackling this issue.

From what I understand, the current implementation handles simple inequalities and symy.Or combinations, but struggles with symy.And expressions. I'm planning to:

1. Add recursive handling for symy.And expressions
2. Ensure proper combination of crossing time conditions
3. Add test cases for complex trigger expressions

Does this sound good? Let me know if there's a particular approach you'd prefer or any edge cases I should keep in mind.

Thanks!