updating get_raster for V3

[natemiller]

This PR updates the get_raster function so that v2 functionality works with v3 of the APIs.

Main changes:

• changed csv parameter to CSV
• changed the url path for pulling region ids from user-context-layers to context-layers
• for consistency with other API parameters, capitalized all references to EEZ, MPA, and RFMO
• Updated raster section README.Rmd so it was compatible with v3.

This PR does not add any functionality not available in v2