Fixed bug (a string passed to a dseries object can be a date).

(cherry picked from commit add3b1401fcfa97e81e3ae1fd71029fadc6b42ff)
Fixed bug. Allow selection of an observation with a date defined in a formatted string.

(cherry picked from commit 3534c68be64288bb949eeea7cfc6a24a85b305d0)
Implement broadcasting for operations (+,-,* and /) between dseries and scalar or vectors. Efficiency and cosmetic changes.

(cherry picked from commit 117f338effd2dd64021666a3f1bc610058c0b7e8)
Allow assignation of variables in an empty dseries object.

(cherry picked from commit 8b093aecbb442d73896bc47f6a2c5a898e4b6243)
Fixed typo in manual.

(cherry picked from commit f11f44fcc573e2b3108ff32bde8c49b3523b81f7)
Update manual according to commit 117f338effd2dd64021666a3f1bc610058c0b7e8.

(cherry picked from commit 0297237dcd643043773c185d10d716e418910415)