Handle irregular and mixed grid types

[rafaqz]

Grid type is specified per dimension, so that arbitrary combinations of regular and irregular indexing are possible. Lat/Lon may need rotation/transformation while time or a vertical index are regularly spaced.

• Transformed grids
• Categorical grids
• Matrix lookup grids
• Cell size field for regular grids - updated on reducing methods
• Rethink Order for irregular and categorical grids

[codecov-io]

Codecov Report

Merging #11 into master will decrease coverage by 4.09%.
The diff coverage is 54.2%.

@@            Coverage Diff            @@
##           master      #11     +/-   ##
=========================================
- Coverage   65.43%   61.34%   -4.1%     
=========================================
  Files          10       10             
  Lines         298      357     +59     
=========================================
+ Hits          195      219     +24     
- Misses        103      138     +35

Impacted Files	Coverage Δ
src/DimensionalData.jl	100% <ø> (ø)	⬆️
src/array.jl	41.86% <0%> (-2.05%)	⬇️
src/interface.jl	47.05% <0%> (-2.95%)	⬇️
src/grid.jl	30.55% <30.55%> (ø)
src/dimension.jl	58% <50%> (-3.71%)	⬇️
src/primitives.jl	76% <70.27%> (-2.05%)	⬇️
src/selector.jl	80.43% <78.94%> (-4.19%)	⬇️

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