Extract data from SoilGrids data files

[bpbond]

Hi @kendalynnm

This PR adds code to extract, for each row of meta_df, the matching clay_15-30cm_mean and ocs_0-30cm_mean data from SoilGrids. Note that

• This is a slow step, so I've added cache=TRUE to the new code chunk
• I've set it to take the mean of a 1000 m radius around each coordinate
• There's one row that has a location mistake - see here, it's the 4564 Sowerby study. You'll want to fix that, as right now it's located in the middle of the ocean (and so no clay or carbon values are extracted)

Data files: you should put them into the new folder soilgrids/. They are very large and I will send you a link by email.

Closes #50

[bpbond]

@kendalynnm This is waiting on your review 😄