Add reverse translation

[jakobnissen]

Implement reverse translation

This PR implements reverse translation. That is, given an AminoAcid and a reverse translation table (defaulting to the standard genetic code), return the set of possible RNACodon that codes for that amino acid. Alternatively, given an AASeq, return a vector of these sets, one seq per amino acid.

Implements #19

Based on #24 . Review and merge/close that one before reviewing this.

Types of changes

This PR implements the following changes:

• [X] :sparkles: New feature (A non-breaking change which adds functionality).

:clipboard: Additional detail

The sets are implemented as a new type, CodonSet, which is a lightweight AbstractSet{RNACodon} with just a few basic methods implemented.

Reverse translation is dependent on the creation of ReverseGeneticCode objects, which are tables, similar to GeneticCode objects. The PR defines const rev_standard_genetic_code. In order not to bloat the package, no other genetic codes are defined by default, but the user can create one using e.g. my_rev_code = ReverseGeneticCode(my_genetic_code).

Discussion points

• Does this PR belong in this package?
• Should other reverse genetic codes be defined by default?
• Should reverse translating AA_Gap throw an error (current behaviour), or return an empty CodonSet? What about reverse translating any amino acid which is coded for in a genetic code?

:ballot_box_with_check: TODO

• [x] Allow reverse translating a single AA
• [x] Add docstrings
• [x] Add documentation
• [x] Add unit tests
• [x] Get unit test coverage to 100% of the new code

[codecov[bot]]

Codecov Report

Merging #20 (320439b) into master (320439b) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 320439b differs from pull request most recent head f7c6337. Consider uploading reports for the commit f7c6337 to get more accurate results

@@           Coverage Diff           @@
##           master      #20   +/-   ##
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  Coverage   86.52%   86.52%           
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  Files          12       12           
  Lines         527      527           
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  Hits          456      456           
  Misses         71       71           

Flag	Coverage Δ
unittests	86.52% <0.00%> (ø)

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[jakobnissen]

Rebased on #24 - do not merge this until #24 has been decided for or against. @SabrinaJaye for ease of reviewing, review #24 first. If it's merged, this PR will be smaller.

[jakobnissen]

Now rebased on master