Reorganized routines in subfolders.

src/utilities/convert/date2string.m

src/utilities/convert/freq2string.m

src/utilities/convert/string2date.m

src/utilities/convert/string2freq.m

src/utilities/is/isdate.m

src/utilities/is/isdates.m

src/utilities/is/isfreq.m

src/utilities/is/ismonthly.m

src/utilities/is/isquaterly.m

src/utilities/is/isstringdate.m

src/utilities/is/issubperiod.m

src/utilities/is/isweekly.m

src/utilities/is/isyearly.m

src/utilities/op/add_periods_to_array_of_dates.m

src/utilities/op/add_periods_to_date.m