Swap meaning of is_alternative and holds_alternative

[eseiler]

Resolves seqan/product_backlog#314

[vercel]

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[SGSSGene]

👍

[codecov]

Codecov Report

Merging #2596 (8b70ee7) into master (8eeab86) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2596   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files         271      271           
  Lines       10623    10623           
=======================================
  Hits        10437    10437           
  Misses        186      186           

Impacted Files	Coverage Δ
...ude/seqan3/alphabet/composite/alphabet_variant.hpp	100.00% <100.00%> (ø)

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