Promotion for DNA/RNA sequences

[TransGirlCodes]

This PR will add promotion to biological sequences in order to allow completion of #436.

Essentially the rule will be if you have DNA or RNA sequences and want to promote them to the same type, they will be promoted to the alphabet that is capable of representing all the possible elements, so promote_rule(DNAAlphabet{2}, DNAAlphabet{4}) will return DNAAlphabet{4}, as DNAAlphabet{4} is the one that is capable of representing all elements of DNAAlphabet{2} and DNAAlphabet{4}.

[codecov-io]

Codecov Report

Merging #457 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #457      +/-   ##
==========================================
+ Coverage   70.85%   70.86%   +0.01%     
==========================================
  Files         142      142              
  Lines       10440    10441       +1     
==========================================
+ Hits         7397     7399       +2     
+ Misses       3043     3042       -1

Impacted Files	Coverage Δ
src/seq/Seq.jl	0% <ø> (ø)	:arrow_up:
src/seq/bioseq/operators.jl	91.91% <ø> (ø)	:arrow_up:
src/seq/alphabet.jl	57.4% <100%> (+0.8%)	:arrow_up:
src/seq/bioseq/conversion.jl	66.66% <100%> (+1.66%)	:arrow_up:
src/seq/sequence.jl	86.59% <0%> (-0.14%)	:arrow_down:
src/seq/bioseq/indexing.jl	100% <0%> (ø)	:arrow_up:
src/seq/bioseq/copying.jl	84.33% <0%> (+0.19%)	:arrow_up:
src/intervals/interval.jl	93.24% <0%> (+1.35%)	:arrow_up:

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[TransGirlCodes]

Tests are passing, I'd like to merge in the next hour or so if that's ok.

[bicycle1885]

👍