Improvements to integer typing and overflow protection.

[LTLA]

• Use the sanisizer library to avoid integer overflow, mostly during vector allocations but also in some additions and casts from float.
• Slap const'ness on as many variables as possible, for safety.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
include/umappp/Status.hpp	88.57% <100.00%> (ø)
include/umappp/combine_neighbor_sets.hpp	95.34% <100.00%> (+0.11%)	⬆️
include/umappp/find_ab.hpp	98.41% <100.00%> (+0.10%)	⬆️
include/umappp/initialize.hpp	96.77% <100.00%> (-0.11%)	⬇️
include/umappp/optimize_layout.hpp	97.08% <100.00%> (+0.32%)	⬆️
include/umappp/parallelize.hpp	100.00% <100.00%> (ø)
include/umappp/spectral_init.hpp	95.08% <100.00%> (-0.08%)	⬇️
tests/src/optimize_layout.cpp	100.00% <100.00%> (ø)

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