dseries submodule update

— add bi-annual frequency
— do not return NaNs if a variable is 0 in all periods
— fixed bug (dates member was wrong)

dseries @ 5ae52153

-Subproject commit a4479b5cb56b0f2b83d567652735c5410254f0fb
+Subproject commit 5ae52153303b71a256b8593cca10e4777e4b5d26