add filters to get_raster

[natemiller]

This PR addresses issue #97 allowing 4wings rasters to be filtered by geartype or flag. This involves adding a filter_by parameter to the get_raster function.

Examples:

Filter by flag

key = get_auth()

code_eez <- get_region_id(region_name = 'CIV', region_source = 'eez', key = key)

get_raster(spatial_resolution = 'low',
            temporal_resolution = 'yearly',
            group_by = 'flag',
            date_range = '2021-01-01,2021-10-01', 
            filter_by = "flag IN ('SLV')",
            region = code_eez$id,
            region_source = 'eez',
            key = key)

Filter by geartype

key = get_auth()

code_eez <- get_region_id(region_name = 'CIV', region_source = 'eez', key = key)

get_raster(spatial_resolution = 'low',
            temporal_resolution = 'yearly',
            group_by = 'flag',
            date_range = '2021-01-01,2021-10-01', 
            filter_by = "geartype in ('tuna_purse_seines','driftnets')",
            region = code_eez$id,
            region_source = 'eez',
            key = key)