Don't require element symbols to match symbol_length

[awvwgk]

Constructor could fail due to symbol_length usage in the sorting step.

[codecov]

Codecov Report

Merging #22 (37c90f9) into main (e9b9348) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   64.50%   64.52%   +0.02%     
==========================================
  Files          49       49              
  Lines        4175     4175              
  Branches     1357     1357              
==========================================
+ Hits         2693     2694       +1     
  Misses        553      553              
+ Partials      929      928       -1     

Impacted Files	Coverage Δ
test/testsuite_structure.f90	34.92% <ø> (ø)
src/mctc/io/symbols.f90	71.42% <100.00%> (ø)
src/mctc/io/structure.f90	53.44% <0.00%> (+1.72%)	⬆️

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