Fixed copyright notices.

src/@dseries/align.m

src/@dseries/baxter_king_filter.m

src/@dseries/center.m

src/@dseries/cumprod.m

src/@dseries/cumsum.m

src/@dseries/eq.m

src/@dseries/exist.m

src/@dseries/extract.m

src/@dseries/hpcycle.m

src/@dseries/hptrend.m

src/@dseries/insert.m

src/@dseries/isequal.m

src/@dseries/isnan.m

src/@dseries/mrdivide.m

src/@dseries/ne.m

src/@dseries/plot.m

src/@dseries/plus.m

src/@dseries/private/default_name.m

src/@dseries/set_names.m

src/@dseries/std.m