Fix softclips causing Variant construction errors

[MillironX]

Types of changes

This PR implements the following changes:
(Please tick any or all of the following that are applicable)

• ✨ New feature (A non-breaking change which adds functionality).
• 🐛 Bug fix (A non-breaking change, which fixes an issue).
• 💥 Breaking change (fix or feature that would cause existing functionality to change).

📋 Additional detail

This is a direct fix for #25

Before

julia> using BioAlignments, BioSequences, SequenceVariation

julia> bad_aln = PairwiseAlignment(AlignedSequence(dna"GATTACAAAA", Alignment("7=3S", 1, 1)), dna"GATTACA")
PairwiseAlignment{LongSequence{DNAAlphabet{4}}, LongSequence{DNAAlphabet{4}}}:
  seq:  1 GATTACAAAA 10
          |||||||   
  ref:  1 GATTACA---  7


julia> Variant(bad_aln)
Variant{LongSequence{DNAAlphabet{4}}, DNA} with 1 edit:Error showing value of type Variant{LongSequence{DNAAlphabet{4}}, DNA}:
ERROR: ArgumentError: Invalid variant
...

This PR

julia> using BioAlignments, BioSequences, SequenceVariation

julia> bad_aln = PairwiseAlignment(AlignedSequence(dna"GATTACAAAA", Alignment("7=3S", 1, 1)), dna"GATTACA")

PairwiseAlignment{LongSequence{DNAAlphabet{4}}, LongSequence{DNAAlphabet{4}}}:
  seq:  1 GATTACAAAA 10
          |||||||   
  ref:  1 GATTACA---  7


julia> Variant(bad_aln)
Variant{LongSequence{DNAAlphabet{4}}, DNA} with 0 edit:

☑️ Checklist

• 🎨 The changes implemented is consistent with the julia style guide.
• 📘 I have updated and added relevant docstrings, in a manner consistent with the documentation styleguide.
• 📘 I have added or updated relevant user and developer manuals/documentation in docs/src/.
• 🆗 There are unit tests that cover the code changes I have made.
• 🆗 The unit tests cover my code changes AND they pass.
• 📝 I have added an entry to the [UNRELEASED] section of the manually curated CHANGELOG.md file for this repository.
• 🆗 All changes should be compatible with the latest stable version of Julia.
• 💭 I have commented liberally for any complex pieces of internal code.

[codecov]

Codecov Report

Base: 60.47% // Head: 61.71% // Increases project coverage by +1.24% 🎉

Coverage data is based on head (4d02616) compared to base (3ed27f0).
Patch coverage: 90.90% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   60.47%   61.71%   +1.24%     
==========================================
  Files           1        1              
  Lines         253      256       +3     
==========================================
+ Hits          153      158       +5     
+ Misses        100       98       -2     

Flag	Coverage Δ
unittests	61.71% <90.90%> (+1.24%)	⬆️

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

Impacted Files	Coverage Δ
src/SequenceVariation.jl	61.71% <90.90%> (+1.24%)	⬆️

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