Various cleanups

[jmert]

• Make boundschecking upon the normalization object part of the interface.
  • Documented the interface.
• Fix an error in handling the internal type promotion mechanism.
• Move some tests from part of the interface test suite back into the primary implementation tests.
• Simplified/consolidated various tests.
• Split up the documentation into smaller units.

[codecov[bot]]

Codecov Report

Merging #4 into master will increase coverage by 0.44%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   97.17%   97.61%   +0.44%     
==========================================
  Files           9        9              
  Lines         248      252       +4     
==========================================
+ Hits          241      246       +5     
+ Misses          7        6       -1     

Impacted Files	Coverage Δ
src/Legendre.jl	100.00% <ø> (ø)
src/broadcasting.jl	100.00% <100.00%> (ø)
src/calculation.jl	97.58% <100.00%> (+0.73%)	:arrow_up:
src/interface.jl	66.66% <100.00%> (+16.66%)	:arrow_up:
src/norm_table.jl	94.28% <100.00%> (+0.73%)	:arrow_up:
src/scalar.jl	100.00% <100.00%> (ø)

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