raise an informative error if provided with (old-style) SLiMMutationModel mutations

[petrelharp]

Apparently, adding mutations using msprime.SLiMMutationModel will not raise any errors until in convert_alleles notices that some of the mutation IDs are negative. We should catch this situation earlier and throw an informative error.

[petrelharp]

Hm: we could change the metadata schema so that mutation IDs must be nonnegative, by setting "minimum" : 0. That would have the advantage that old-style msprime mutations would just fail to validate at all, so you'd get an error like:

tskit.exceptions.MetadataValidationError: -187961329834 is less than the minimum of 0

On instance['derived_states']:
    -187961329834

I'm not sure when you'd get this error. I think that you'd get it the next time you tried to look at the mutation in question, which would in the normal course of things happen during pyslim.add_mutation_metadata, and so we could catch the error and provide a useful suggestion. However, if the user also missed out pyslim.add_mutation_metadata they wouldn't get it (and nothing we could do there). So, this seems like a good way to go? However, in my testing right now, it seems like maybe the minimum key isn't working on the mutation metadata schema? (It's working elsewhere with the struct codec though.)

[bhaller]

I would again suggest changing the name to msprime.SLiMv5MutationModel to make people think about what they are doing. Breaking people's code is sometimes good.

[petrelharp]

Hm. Gee, I wish that we'd discussed that suggestion before I did those last two SLiM releases.

[bhaller]

Hm. Gee, I wish that we'd discussed that suggestion before I did those last two SLiM releases.

I thought I pushed that fairly hard and got voted down...?