utils: add 'get_n_series' and 'get_series_slice'
Those two functions are utils to know how many series exists from a list of ImageKey and get serie limits (as a slice) from a list of ImageKey, a type and an index.
Those two functions are utils to know how many series exists from a list of ImageKey and get serie limits (as a slice) from a list of ImageKey, a type and an index.