assert_capture_matrix() is an internal helper function aiding with the
checking of an internal R object that contains the parsing results as
returned by parse_dttm(): capture matrix.
This function checks that the capture matrix is a matrix and that it contains
six columns: year, mon, mday, hour, min and sec.
assert_capture_matrix(m)This function throws an error if m is not either:
A character matrix;
A matrix whose columns are (at least): year, mon, mday, hour,
min and sec.
Otherwise, it returns m invisibly.
A character matrix.