Move combine_hash function to tools/hash_utils

[mmore500]

Also, write tests for combine_hash. Note that this breaks
existing levilization described in README.md... now meta
depends on tools. Update README.md to acknowledge this.

[mmore500]

This change needs some thinking RE: levilization.

[codecov]

Codecov Report

Merging #205 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #205      +/-   ##
==========================================
- Coverage   65.05%   65.04%   -0.02%     
==========================================
  Files          95       95              
  Lines       13153    13215      +62     
==========================================
+ Hits         8557     8596      +39     
- Misses       4596     4619      +23

Impacted Files	Coverage Δ
source/tools/hash_utils.h	100% <100%> (ø)	⬆️
tests/test_tools.cc	99.37% <100%> (+0.02%)	⬆️
source/meta/meta.h	54.38% <100%> (-0.79%)	⬇️
source/hardware/AvidaGP.h	36.01% <0%> (-11.38%)	⬇️
source/hardware/InstLib.h	32.81% <0%> (-3.13%)	⬇️
tests/test_systematics.cc	96.57% <0%> (+0.28%)	⬆️
source/hardware/AvidaCPU_InstLib.h	49.01% <0%> (+1.96%)	⬆️

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