Improve heuristics of translating ambiguous nucs

[jakobnissen]

Use a more principled approach, such that we now guarantee that ambiguous DNA
and RNA is translated to exactly the set of amino acids the genetic code
corresponds to.

[codecov]

Codecov Report

Patch coverage: 87.50% and project coverage change: -0.47 ⚠️

Comparison is base (1ccb602) 91.21% compared to head (d5f81d6) 90.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
- Coverage   91.21%   90.75%   -0.47%     
==========================================
  Files          31       31              
  Lines        2403     2401       -2     
==========================================
- Hits         2192     2179      -13     
- Misses        211      222      +11     

Flag	Coverage Δ
unittests	90.75% <87.50%> (-0.47%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/geneticcode.jl	80.53% <87.50%> (-6.32%)	⬇️

... and 3 files with indirect coverage changes

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