bug fixes in add_mutation_metadata_tables

[jeffspence]

I've been trying to use pyslim in the multi-trait addition to stdpopsim and was getting some errors related to pyslim.add_mutation_metadata_tables . Digging into it, there were two separate things going on:

1. If the tables object passed to pyslim.add_mutation_metadata_tables has multiple traits, that information is not passed into default_slim_metadata(), which then results in errors resulting from mismatching dimensions. Adding num_traits=len(ts_metadata["SLiM"]["traits"]) as a kwarg to default_slim_metadata() fixes that.
2. if mut_ids is empty, then np.array(mut_ids, dtype="int") only has one dimension (of length 0), but subsequent code assumes that it is 2 dimensional. Wrapping the code to extend ts_metadata["SLiM_mutation_list"] in an if block deals with this, and shouldn't affect anything because if mut_ids is empty, then there's nothing to add to ts_metadata["SLiM_mutation_list"] anyway.

I didn't write any tests for these because I'm not sure where to start (and a bunch of the tests are currently failing independent of the code changes proposed in this PR) but I'd be happy to do that with a bit of guidance.

Edit: I also found a missing word in the convert_alleles docstring.