feat: Allow custom file names in the exposure workflow

[carlosgjs]

The following file names are hard-coded in scripts/exposure_workflow.R:

historical_climate <- readRDS(file.path(path, "historical_climaate_data.rds"))
future_climate     <- readRDS(file.path(path, "future_climaate_data.rds"))
grid               <- readRDS(file.path(path, "grid.rds"))
primates_shp       <- readRDS(file.path(path, "primates_shapefiles.rds"))