fix allele frequency calculation in slim code, or disallow multiple drawn mutations per MutationType

[grahamgower]

The allele frequency calculation in the slim engine (used for allele frequency conditioning) currently assumes there is only one mutation per MutationType.

stdpopsim/stdpopsim/slim_engine.py

Lines 444 to 452 in 973d5b0

	// Return the allele frequency of a drawn mutation in the specified population.
	// Assumes there's only one mutation of the given type.
	function (float$)af(object$ mut_type, object$ pop) {
	mut = sim.mutationsOfType(mut_type);
	if (length(mut) == 0) {
	return 0.0;
	}
	return sim.mutationFrequencies(pop, mut);
	}

But a user could easily break this by drawing two mutations of the same mutation type. See note here:

stdpopsim/stdpopsim/ext/selection.py

Lines 115 to 133 in 973d5b0

	@attr.s(kw_only=True)
	class DrawMutation(ExtendedEvent):
	"""
	TODO: docstring.
	Draw a new mutation with the given mutation type in the given population
	at the given genomic coordinate. The mutation will be added to one randomly
	chosen chromosome in the given population.
	If save=True, the simulation state is saved before the mutation is
	introduced, to facilitate the save/restore mechanism used for allele
	frequency conditioning.
	FIXME: Drawing multiple mutations using the same mutation type causes
	erroneous allele frequency calculation in the SLiM code!
	Multiple drawn mutations should be disallowed, or the allele frequency
	calculation should somehow be tied to specific drawn mutations.
	See SLiM documentation for addNewDrawnMutation().
	"""

[nspope]

Fixed by #1341. We're opting to disallow multiple DrawMutation events at the same site for the time being.