Add findfirst method for BioSymbol

[jakobnissen]

See #141

This PR adds findfirst and findlast methods of the type findfirst(::BioSymbol, ::BioSequence). How it currently works is:

• findfirst(::BioSymbol, ::BioSequence) allows for compatible but different symbols, i.e. dna"TACNA" has a potential G at position 4. This is the new thing in the PR.
• findfirst(::BioSequence, ::BioSequence) likewise allows compatible symbols, just like before.
• To get literal match against a symbol, do findfirst(isequal(DNA_A), dna"GGA")
• There is currently no way to search for literal matches between two biological sequences. Perhaps there should be, but I'm not sure what the syntax should be.

Issues / things to mull over:

• Currently, search uses compatbits for the element BioSymbol type, and this is not generically defined for user-defined biosymbols (see Does this pakcage need an interface? BioSymbols.jl#42).
• There is no way to search for literal biosequence matches.

[codecov]

Codecov Report

Merging #166 (e7cec9d) into v3 (87f806c) will increase coverage by 0.03%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##               v3     #166      +/-   ##
==========================================
+ Coverage   82.14%   82.17%   +0.03%     
==========================================
  Files          31       31              
  Lines        2156     2171      +15     
==========================================
+ Hits         1771     1784      +13     
- Misses        385      387       +2     

Flag	Coverage Δ
unittests	82.17% <85.71%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
src/search/exact.jl	91.96% <85.71%> (-0.90%)	⬇️
src/BioSequences.jl	0.00% <0.00%> (ø)
src/longsequences/longsequence.jl	100.00% <0.00%> (ø)
src/biosequence/predicates.jl	98.07% <0.00%> (+0.03%)	⬆️

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[TransGirlCodes]

I think this is good.

As for your question as to a non-compatibility version of findfirst(::BioSequence, ::BioSequence), in the future.

Have you considered having the method accept an optional keyword that toggles the behaviour between finding the first compatible subsequence, and the first exact subsequence? Maybe say if strict = true then the symbols subsequence must be exact, not just compatible.

[jakobnissen]

Yes, that sounds like a good idea. I'll see if I can make a PR.