Standardise `getraster`

[rafaqz]

This PR further standardises the form and output of getraster.

• only the getraster(T, layers; kw...) form is exported
• Symbol or Int layers return a String
• Tuple of layers returns a NamedTuple of String
• month and date keywords can be Vector. These wrap the previous outputs in Vector.
• date is always the outer Vector, month the inner. (we can have climate for a particular time period, so there is the date of the time period, and the cimate months as separate things).
• No other keyword arguments can be a Vector, without consideration, and updating this schema in the getraster docstring.

This should make it easier for scripts and external packages to handle more variation in datasets and keywords.

[codecov[bot]]

Codecov Report

Merging #37 (96e16b5) into master (5e46950) will decrease coverage by 0.25%. The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #37      +/-   ##
==========================================
- Coverage   73.66%   73.41%   -0.26%     
==========================================
  Files          15       15              
  Lines         357      395      +38     
==========================================
+ Hits          263      290      +27     
- Misses         94      105      +11     

Impacted Files	Coverage Δ
src/worldclim/weather.jl	18.18% <0.00%> (-1.82%)	:arrow_down:
src/types.jl	54.54% <28.57%> (-45.46%)	:arrow_down:
src/awap/awap.jl	82.75% <37.50%> (-13.54%)	:arrow_down:
src/alwb/alwb.jl	92.85% <75.00%> (-7.15%)	:arrow_down:
src/shared.jl	62.85% <83.33%> (-1.43%)	:arrow_down:
src/chelsa/future.jl	62.12% <88.23%> (+4.64%)	:arrow_up:
src/chelsa/bioclim.jl	100.00% <100.00%> (+9.09%)	:arrow_up:
src/earthenv/habitatheterogeneity.jl	93.10% <100.00%> (+0.24%)	:arrow_up:
src/earthenv/landcover.jl	100.00% <100.00%> (ø)
src/worldclim/bioclim.jl	100.00% <100.00%> (+4.76%)	:arrow_up:
... and 1 more

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