bugfix and improve srtm

[rafaqz]

This PR does a few things.

First it checks if there are existing tiles in the SRTM dataset (e.g. there are many missing over the ocean), and otherwise returns missing.

It also allows tuple of tuple syntax for bounds (as in Rasters.jl), and automatically sorts the bounds to cover an actual area. Maybe the last thing is unnecessary. @jenkspt if you want to take a look at this that would be helpful.

[jenkspt]

I would think we just throw an error if the bounds is in the wrong order -- rather than sorting them. Everything else looks good!

[codecov[bot]]

Codecov Report

Merging #50 (6c6f130) into master (29d185a) will decrease coverage by 0.79%. The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   75.00%   74.20%   -0.80%     
==========================================
  Files          16       15       -1     
  Lines         464      469       +5     
==========================================
  Hits          348      348              
- Misses        116      121       +5     

Impacted Files	Coverage Δ
src/srtm/srtm.jl	88.88% <93.75%> (-3.42%)	:arrow_down:
src/types.jl	71.42% <0.00%> (-7.15%)	:arrow_down:
src/earthenv/landcover.jl	96.29% <0.00%> (-3.71%)	:arrow_down:
src/worldclim/weather.jl	14.70% <0.00%> (-2.95%)	:arrow_down:
src/RasterDataSources.jl

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