issymmetric/ishermitian for OneElement

[jishnub]

This adds O(1) symmetric/hermitian checks for OneElement.
After this,

julia> O = OneElement(3, (5,5), (1000,1000));

julia> @btime issymmetric($O);
  3.109 ns (0 allocations: 0 bytes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (5b31642) to head (149f113).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #380   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files           8        8           
  Lines        1121     1123    +2     
=======================================
+ Hits         1120     1122    +2     
  Misses          1        1           

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