Improve selectors

[rafaqz]

Test and improve selector behaviour, including adding tolerances to At selectors. The default eps based tolerance of rtol fixes missed matches due to floating point errors in LinRange.

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 5.79%.
The diff coverage is 77.61%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   69.37%   75.16%   +5.79%     
==========================================
  Files          11       11              
  Lines         431      447      +16     
==========================================
+ Hits          299      336      +37     
+ Misses        132      111      -21

Impacted Files	Coverage Δ
src/DimensionalData.jl	100% <ø> (ø)	⬆️
src/array.jl	65.11% <0%> (+10.76%)	⬆️
src/primitives.jl	78.66% <75%> (+6.55%)	⬆️
src/grid.jl	80.35% <78.94%> (+19.57%)	⬆️
src/selector.jl	80.7% <80.95%> (-1.91%)	⬇️
src/interface.jl	46.66% <0%> (-6.67%)	⬇️
src/dimension.jl	79.03% <0%> (+1.61%)	⬆️

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