Move some indexing logic from LongSequence to BioSequence

[jakobnissen]

This simply moves some of the general indexing logic from LongSequence to BioSequence, to make it more general. For example, you can now do

julia> a = dna"TAGC";

julia> a[2:4] = mer"ATG";

julia> a
4nt DNA Sequence:
TATG

[codecov]

Codecov Report

Merging #133 (d0fef87) into master (2527208) will decrease coverage by 0.51%.
The diff coverage is 60.52%.

@@            Coverage Diff             @@
##           master     #133      +/-   ##
==========================================
- Coverage   83.04%   82.52%   -0.52%     
==========================================
  Files          45       45              
  Lines        3061     3073      +12     
==========================================
- Hits         2542     2536       -6     
- Misses        519      537      +18     

Flag	Coverage Δ
unittests	82.52% <60.52%> (-0.52%)	⬇️

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

Impacted Files	Coverage Δ
src/biosequence/indexing.jl	70.42% <21.05%> (-23.92%)	⬇️
src/longsequences/indexing.jl	100.00% <100.00%> (ø)
src/longsequences/seqview.jl	93.54% <100.00%> (ø)

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