Add bi-annual frequency.

src/@dates/dates.m

src/@dates/subperiod.m

src/@dseries/dseries.m

src/@dseries/hdiff.m

src/@dseries/hdiff_.m

src/@dseries/hgrowth.m

src/@dseries/hgrowth_.m

src/@dseries/mdiff_.m

src/@dseries/mgrowth_.m

src/@dseries/qdiff_.m

src/@dseries/qgrowth_.m

src/@dseries/subsref.m

src/@dseries/ydiff_.m

src/@dseries/ygrowth_.m

src/utilities/convert/date2string.m

src/utilities/convert/freq2string.m

src/utilities/convert/string2date.m

src/utilities/convert/string2freq.m

src/utilities/is/isbiannual.m

src/utilities/is/isfreq.m