Multiple merger polyploid scaling

[JereKoskela]

Ping @jeromekelleher. Adding tests for polyploid scaling for multiple merger coalescents turned out to be easy, but also sent me down a bit of a rabbit hole. Summary of changes:

• The extra spaces on various lines in msprime.c which are unrelated to anything I've done (1396, 1899...) were put in by clang. Let me know if you want me to do something about them.
• For the Dirac coalescent, population growth at rate g should yield a coalescent growth rate of 2g,essentially because the coalescence probability in the Moran model is 1/N^2 rather than 1/N. We had overlooked that, but it is now fixed.
• Ploidy wasn't being handled quite right in the Dirac coalescent either, but is now fixed. In the process, I got rid of some if (ploidy == 1) {...} else {...} statements which became redundant.
• For both the Dirac and Beta coalescents, I moved the allocation of the ancestor buckets Q later into the rejection sampler. It is now only done once a multiple merger event is definitely taking place.
• The Dirac coalescent test in test_ancestry.py started failing after the ploidy fix because the rate of multiple mergers wasn't high enough to guarantee one anymore. I increased it.
• I also noticed we were disallowing psi = 1 in the Dirac coalescent for no reason. The documented (and sensible) range is 0 < psi <= 1. I tweaked the boundary values and associated tests to make the code consistent with the documentation.

[codecov]

Codecov Report

Attention: Patch coverage is 85.45455% with 8 lines in your changes missing coverage. Please review.

Project coverage is 90.98%. Comparing base (7492287) to head (42ec186).

Files	Patch %	Lines
lib/msprime.c	84.90%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2309      +/-   ##
==========================================
- Coverage   90.99%   90.98%   -0.02%     
==========================================
  Files          20       20              
  Lines       12035    12021      -14     
  Branches     2441     2437       -4     
==========================================
- Hits        10951    10937      -14     
  Misses        599      599              
  Partials      485      485              

Flag	Coverage Δ
C	90.98% <85.45%> (-0.02%)	⬇️
python	98.70% <ø> (ø)

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[jeromekelleher]

Sorry, making a mess of git stuff here, I'll fix it up