extractBhatt does not work discrete covariates

[laurence-hudson-tessella]

For discrete covariates extractBhatt attempts to run extractAdmin for gaul/polygon occurrences with fun = 'modal'. This argument is passed to library(raster)'s zonal method. 'modal' is not a supported argument for zonal, R's modal (no quotes) function could work, but covariate stacks aligned to the SEEG master grid are too large for this. One option might be to use modal against one covariate layer at a time.

[laurence-hudson-tessella]

The "one discrete covariate at a time" workaround is implemented in abraidBhatt/extractBatch, but performance is poor.