Improve LabeledArray and documentation

[junyuan-chen]

The changes are mostly on LabeledValue and LabeledArray:

1. The violation of transitivity of == discussed here has been fixed by removing methods that allow direct comparisons with Strings.
2. The element type of data array and the key type of the value label dictionary are allowed to be different.
3. More methods on indexing and array operations are added for LabeledArray.
4. Phrasing of value labels is more careful to avoid confusion with variable labels. The method labels is renamed to valuelabels as the former will refer to variable labels, in anticipation of the future release of TableMetadataTools.jl.

[codecov]

Codecov Report

Merging #11 (271c02f) into main (26c9daf) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #11   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          624       656   +32     
=========================================
+ Hits           624       656   +32     

Impacted Files	Coverage Δ
src/ReadStatTables.jl	100.00% <ø> (ø)
src/datetime.jl	100.00% <ø> (ø)
src/LabeledArrays.jl	100.00% <100.00%> (ø)
src/parser.jl	100.00% <100.00%> (ø)
src/table.jl	100.00% <100.00%> (ø)

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