add WorldClim Elevation getraster_keyword method

[rafaqz]

missed this earlier

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 :tada:

Comparison is base (9a3752e) 72.22% compared to head (3a82753) 72.25%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #59 +/- ## ========================================== + Coverage 72.22% 72.25% +0.03% ========================================== Files 19 19 Lines 738 739 +1 ========================================== + Hits 533 534 +1 Misses 205 205 ``` | [Impacted Files](https://codecov.io/gh/EcoJulia/RasterDataSources.jl/pull/59?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EcoJulia) | Coverage Δ | | |---|---|---| | [src/worldclim/elevation.jl](https://codecov.io/gh/EcoJulia/RasterDataSources.jl/pull/59?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EcoJulia#diff-c3JjL3dvcmxkY2xpbS9lbGV2YXRpb24uamw=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EcoJulia). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EcoJulia)

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