Refactor SoilGrids functions

[dlebauer]

Description

This is motivated by

• a discussion in Add codes to extract SoilGrids soil texture data and derive ensemble … #3406 that included
• need to add coarse fraction to estimation of carbon stocks
• Results of benchmark testing that show downloading first then extracting points is 100x faster than querying the API for each point.
• Generalizing functions to work with any soilgrids variable.

Changes will:

• Improve the efficiency and scalability of SoilGrids queries, especially for large datasets.
• Simplify and generalize the workflow to handle multiple SoilGrids variables.

Proposed Solution

Refactor soilgirds_soilC_extract and soilgrids_texture_extraction into three modular functions to increase functionality and flexibility, generalized to work with any variable and depth layer.

• download_soilgrids_raster a function to download tiff files
• query_soilgrids a function to query data from points or polygons from remote VRTs or from local rasters
• soilgrids_uncertainty to estimate uncertainties
• soilgrids_carbonstocks to calculate OCS to any depth from SOC (to generalize soilgrids 0-30 OCS data product to multiple depths)

Alternatives Considered

Status Quo

FYI @mdietze @DongchenZ @Qianyuxuan @infotroph