Refactor CHELSA future from #24, add climate and CMIP6

[rafaqz]

Some refactors to #24 to use a single type parameter, and to allow generalisation to BioClim/Climate and both CMIP5 and CMIP6 future datasets.

@tpoisot I rebased your commits down to a single commit for all of #24 (in your name) because rebasing them all against master was hard and messy.

Pretty much all of CHELSA Future should be accessible from this PR.

[codecov[bot]]

Codecov Report

Merging #34 (412e232) into master (27eceb7) will decrease coverage by 9.29%. The diff coverage is 59.42%.

:exclamation: Current head 412e232 differs from pull request most recent head c83ff7d. Consider uploading reports for the commit c83ff7d to get more accurate results

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   82.95%   73.66%   -9.30%     
==========================================
  Files          12       15       +3     
  Lines         223      357     +134     
==========================================
+ Hits          185      263      +78     
- Misses         38       94      +56     

Impacted Files	Coverage Δ
src/RasterDataSources.jl	100.00% <ø> (ø)
src/shared.jl	64.28% <50.00%> (-3.72%)	:arrow_down:
src/chelsa/future.jl	57.48% <57.48%> (ø)
src/chelsa/bioclim.jl	90.90% <100.00%> (-1.40%)	:arrow_down:
src/chelsa/shared.jl	100.00% <100.00%> (ø)
src/types.jl	100.00% <100.00%> (ø)

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[rafaqz]

A little unsatisfactory not testing all the options, but downloading this many files is too much for CI.

Maybe later we can test if the url exists instead of downloading.