Added unit tests.

data/test_dates4display.m

src/@dates/copy.m

src/@dates/dates.m

src/@dates/end.m

src/@dates/isequal.m

src/@dates/length.m

src/@dates/private/comparison_arg_checks.m

src/@dates/private/dates4display.m

src/@dates/private/greaterorequal.m

src/@dates/private/greaterthan.m

src/@dates/private/lessorequal.m

src/@dates/private/lessthan.m

src/@dates/strings.m

src/@dates/subsasgn.m

src/@dates/vertcat.m

src/utilities/convert/date2string.m

src/utilities/convert/freq2string.m

src/utilities/convert/string2freq.m

src/utilities/is/isdates.m

src/utilities/is/isfreq.m