Add histogram fitting for Raster data structures and break code into submodules

[jbisits]

This PR adds methods to fit from StatsBase.jl for Rasters, RasterStacks and RasterSeriess creates the new module RasterHistogras. There is an abstract type AbstracRasterHistogram and a mutable type for generating a Histogram from a Raster, RasterStack, or RasterSeries. The RasterLayerHistogram is 1 dimensional while the RasterStackHistogram and RasterSeriesHistogram can be N-dimensional where N is the number of layers. The types are mutable so that the Histogram (that is fit using StastBase.jl) can be normalized and updated in the AbstractRasterHistogram.

There are functions to calculate area and volume weights based on the dimensions of Raster source.

The code is also organised into submodules so only parts of the package that are necessary to ones use can be loaded.
Closes #35

[codecov]

Codecov Report

Merging #33 (8a9f708) into main (66a034f) will decrease coverage by 6.19%.
The diff coverage is 89.34%.

@@             Coverage Diff             @@
##              main      #33      +/-   ##
===========================================
- Coverage   100.00%   93.81%   -6.19%     
===========================================
  Files            2        2              
  Lines          123      291     +168     
===========================================
+ Hits           123      273     +150     
- Misses           0       18      +18     

Impacted Files	Coverage Δ
src/oceanvariableconversions.jl	100.00% <ø> (ø)
src/oceanvariabledistributions.jl	89.34% <89.34%> (ø)

... and 1 file with indirect coverage changes

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[jbisits]

Once tests have been updated for the new way the RasterHistograms module is structured codecov will jump back up.

[jbisits]

Codecov is less because of the Base.show methods and convert_arguments. Tests can be written for this later so for now I will merge this. If any other bugs pop up when writing documentation I will fix in new PRs.