Improve bounds handling for grids, add methods for cat, extrema etc

[rafaqz]

Accidentally rebased these into one commit.

[codecov-io]

Codecov Report

Merging #24 into master will increase coverage by 5.29%.
The diff coverage is 82.64%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   72.19%   77.49%   +5.29%     
==========================================
  Files          11       11              
  Lines         464      511      +47     
==========================================
+ Hits          335      396      +61     
+ Misses        129      115      -14

Impacted Files	Coverage Δ
src/DimensionalData.jl	100% <ø> (ø)	⬆️
src/selector.jl	87.93% <100%> (+7.22%)	⬆️
src/dimension.jl	90.9% <100%> (+18.18%)	⬆️
src/grid.jl	80% <75%> (-0.36%)	⬇️
src/methods.jl	80.76% <77.77%> (-2.21%)	⬇️
src/primitives.jl	83.22% <91.17%> (+6.1%)	⬆️

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