Add letters function for PcGroupElem

[jamesnohilly]

Moved the syllable functions from the mjrodgers-OW_GModules branch to this to use as a reference for new work.

• New function letters for (Sub)PcGroupElem.
• A check in exponent_vector for whether the input is really "canonical".
• Test cases in test/Groups/pcgroups.jl for the new functions.
• Add doc strings for new functions.

Suggestions from @fingolfin

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.

Project coverage is 84.58%. Comparing base (da7acb0) to head (e9a4ac4).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Groups/pcgroup.jl	0.00%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4202      +/-   ##
==========================================
- Coverage   84.59%   84.58%   -0.02%     
==========================================
  Files         631      631              
  Lines       84899    84915      +16     
==========================================
  Hits        71824    71824              
- Misses      13075    13091      +16     

Files with missing lines	Coverage Δ
src/Groups/pcgroup.jl	62.58% <0.00%> (-8.15%)	⬇️