Remove MAX_ALLELES=255 restriction

[jeromekelleher]

This is currently an artificial restriction which causes issues in certain extreme cases.

Note that we don't need to efficiently handle very large numbers of alleles; the current algorithm which is efficient for small numbers of alleles is perfectly fine.

cc @bhaller

[bhaller]

Yes, this sounds right. Fixing this is not at all a priority, but it is conceivable that a user trying to do something weird in SLiM might run into it. Mostly, fixing it will just make a couple of tests in my test suite go from red to green. :->