Sympy cannot solve for `t` in PEtab v2 event expressions

[BSnelling]

When a DEModel Event is constructed, Sympy attempts to solve the event trigger expression for t:

AMICI/python/sdist/amici/_symbolic/de_model_components.py

Lines 768 to 772 in 20b07e9

	try:
	self._t_root = sp.solve(self.get_val(), amici_time_symbol)
	except NotImplementedError:
	# the trigger can't be solved for `t`
	pass

)

In the case of PEtab events (as of PEtab v2) the trigger expression takes the form of a piecewise expression e.g.

Min(1/2 - _petab_preequilibration_indicator, _petab_experiment_indicator_e0 - 1/2, t - 10)

Sympy fails to solve expressions like this for $t$ with the following error:

*** NotImplementedError: 
No algorithms are implemented to solve equation Min(1/2 - _petab_preequilibration_indicator, _petab_experiment_indicator_e0 - 1/2, t - 10)

AMICI passes on this error and leaves _t_root empty. So there is no immediate error, but this means the function to check if an event has explicit trigger times does not work as expected.

AMICI/python/sdist/amici/_symbolic/de_model_components.py

Line 843 in 126e936

def has_explicit_trigger_times(

[dweindl]

I have a proof-of-concept for addressing that. However, this will require a bit of refactoring. Depending on the indicator variables, the event above will trigger at $t=10$ or not at all. However, the current setup assumes that we always have an expression for the trigger time. Whether the event can trigger or not can only be decided once the indicator values have been set. Therefore, we will need to handle some sentinel for "doesn't trigger at all".