Disallow unstable beta coalescents

[JereKoskela]

Closes #2256 (again) by restricting the valid region of the Beta-coalescent alpha-parameter to 1 < alpha <= 1.991.

I can't make the timescale near alpha = 1 break even for values around 1 + 1e-15. So I've left that boundary case as it is, restricted only by alpha > 1. I'm amazed that it's so robust!

[codecov[bot]]

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[JereKoskela]

This time I remembered the changelog.

[JereKoskela]

Ping @jeromekelleher & @Sendrowski. I think this is ready to be merged unless you have comments.

[Sendrowski]

It looks good to me!

[jeromekelleher]

I guess we should pop out another point release?