Add guessparse function

[jakobnissen]

This function is a quick-and-dirty parser function from AbstractString to LongSequence, with autodetection of the alphabet. It's meant to be used in ephemeral REPL work, and very clearly documented to be unstable and subject to change.

See #268

This is just a draft. The implementation is straightforward, but we might want to think about whether we want this, and what it should be called.
Preferably, the name should be:

• Short, since it's meant to be used in the REPL
• Very clear in that this function is based on guesswork, and is therefore not suitable for long-lasting code.

TODO:

• Bikeshed the name and functionality
• Tests
• Update changelog
• Update docs
• More comments in the LUT table generation

[codecov]

Codecov Report

❌ Patch coverage is 4.54545% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.58%. Comparing base (4a31474) to head (fdbb494).
⚠️ Report is 46 commits behind head on master.

Files with missing lines	Patch %	Lines
src/longsequences/constructors.jl	5.26%	18 Missing ⚠️
src/alphabet.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #269      +/-   ##
==========================================
- Coverage   91.20%   90.58%   -0.63%     
==========================================
  Files          31       31              
  Lines        2400     2421      +21     
==========================================
+ Hits         2189     2193       +4     
- Misses        211      228      +17     

Flag	Coverage Δ
unittests	90.58% <4.54%> (-0.63%)	⬇️

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[cjprybol]

Valid DNA?

[cjprybol]

This looks great, thanks @jakobnissen! I like having both the guessparse and guess_alphabet functions, as you've done here.

This seems much more efficient than I was initially picturing (using regex or the tryparse approach), which is excellent.

[kescobo]

I wonder if it would be worth looking at CSV.read() for inspiration here. There, the function goes through X lines and tries to guess the type of each column. X is user-selectable but has a sensible default.

I'm wondering if we want to always return a type, or throw an error if it's ambiguous... Maybe as an alternative the the later, offer an optional type to use as a default, like guessparse(seq, LongDNA{2})

For bikeshedding, I quite liked swagparse, but that's not very discoverable... then again, might be worth being silly if we want to signal that this is experimental/unstable