Subsets the metadata table by data 'type', 'city', 'year' and 'mode'
select_metadata(t = NULL, c = NULL, y = NULL, m = NULL)A data.frame object with metadata subsetted by data type,
'city', 'year' and 'mode'
Type of data: 'access' or 'grid' (passed from read_ function)
City (passed from read_ function)
Year of the dataset (passed from read_ function)
Transport mode (passed from read_ function)