Correctly account for isolated samples in normalisation of pair_coalescence_* methods

[nspope]

Fixes #3459

There's the somewhat thorny issue of what the correct normalisation is when only one of span_normalise or pair_normalise are True. In this case (using the notation of the linked issue), what I've done is normalise by the "average number of pairs" $\int p(x) dx / (b - a)$ for "pair normalisation" (so the units are "sequence length per pair") -- this reduces to ${n \choose 2}$ when there's no isolated samples which agrees with the original implementation. Similarly I normalise by the "average pair span" $\int p(x) dx / {n \choose 2}$ for "span normalisation" (so that the units are "# of sample pairs") -- which simplifies to $b - a$ as per the original implementation where there are no isolated samples.

So the output (for the $i$th node or time window) is one of:

• $C_i$ (unnormalised)
• $C_i / \int p(x) dx$ (fully normalised)
• $C_i \times (b - a) / \int p(x) dx$ (pair normalised)
• $C_i \times {n \choose 2} / \int p(x) dx$ (span normalised)

The above assumes that there's no missing trees over the interval--- if there are, replace $b - a$ by "nonmissing span" (the original implementation discounted missing span as well)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.68%. Comparing base (1c689c0) to head (be9f67f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3460      +/-   ##
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+ Coverage   91.67%   91.68%   +0.01%     
==========================================
  Files          38       38              
  Lines       32186    32248      +62     
  Branches     5150     5167      +17     
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+ Hits        29505    29567      +62     
  Misses       2348     2348              
  Partials      333      333              

Flag	Coverage Δ
C	82.27% <92.20%> (+0.04%)	⬆️
c-python	77.59% <97.36%> (+0.05%)	⬆️
python-tests	96.40% <ø> (ø)
python-tests-no-jit	33.20% <ø> (ø)

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Components	Coverage Δ
Python API	98.70% <ø> (ø)
Python C interface	91.23% <ø> (ø)
C library	91.27% <100.00%> (+0.03%)	⬆️

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[nspope]

Added an additional test into the C suite for "ancestor-descendant" sample pairs