constify C stats api

[castedo]

For #844. This builds on top of #920 which did not constify all the "stats" functions in tree.h.

This PR constifies all those stats functions and fixes duplicated typedefs by keeping them defined only once in trees.h.

[AdminBot-tskit]

📖 Docs for this PR can be previewed here

[codecov]

Codecov Report

Merging #939 into main will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #939   +/-   ##
=======================================
  Coverage   93.40%   93.40%           
=======================================
  Files          25       25           
  Lines       20335    20335           
  Branches      824      824           
=======================================
  Hits        18994    18994           
  Misses       1304     1304           
  Partials       37       37           

Impacted Files	Coverage Δ
c/tskit/trees.c	94.75% <100.00%> (ø)
_tskitmodule.c	90.21% <0.00%> (ø)

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[jeromekelleher]

Thanks @castedo - would you mind marking these as "draft" PRs until they're ready for review/merging? It's tricky for me to keep track of what's what and know what GitHub updates I should pay attention to.

[castedo]

Roughly speaking, this PR just constifies all the stats functions in trees.h/c.

Because this PR still does not change the types in trees.h, some stats functions can not yet be constified (e.g. tsk_treeseq_kc_distance). That happens in the next and final PR #940 .

Two functions have const params that are fun, e.g.

int tsk_treeseq_mean_descendants(..., tsk_id_t *const *reference_sets, ... )

This is a fun case where "Mediterranean/romance" noun-adjective order is the only way to express what stays constant. It might be intuitive reading it "in Spanish", e.g. "tsk_id_t puntero constante puntero" where the const is an adjective to the noun to the left, not to the right.

Heads up there's a non-stats tweak:

int tsk_treeseq_init(tsk_treeseq_t *self, const tsk_table_collection_t *tables, ... )

because the tables are copied.

[jeromekelleher]

This is excellent @castedo, a major improvement in documenting this rather tricky API. Some minor comments and questions.

[castedo]

LGTM for merging.

[jeromekelleher]

Let's merge!