Symmetric fix

[evanfields]

Fix helper method inscost for matrix types besides Matrix. In particular solve_tsp should now work for Symmetric inputs.

[codecov-commenter]

Codecov Report

Merging #23 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #23   +/-   ##
=======================================
  Coverage   93.56%   93.56%           
=======================================
  Files           4        4           
  Lines         264      264           
=======================================
  Hits          247      247           
  Misses         17       17           

Impacted Files	Coverage Δ
src/helpers.jl	95.34% <ø> (ø)

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