Draft implementation of new SPR distance

[ms609]

Further improvements anticipated soon – but keen to include an experimental implementation in today's CRAN release

[codecov]

Codecov Report

❌ Patch coverage is 69.54436% with 127 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.72%. Comparing base (cd4faa7) to head (34bfa61).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spr_lookup.cpp	67.93%	42 Missing ⚠️
src/information.h	9.30%	39 Missing ⚠️
R/tree_distance_spr.R	69.60%	38 Missing ⚠️
R/tree_information.R	69.56%	7 Missing ⚠️
src/spr.cpp	98.64%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
- Coverage   97.55%   94.72%   -2.83%     
==========================================
  Files          44       46       +2     
  Lines        3308     3660     +352     
==========================================
+ Hits         3227     3467     +240     
- Misses         81      193     +112     

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