Added new function from.

Allows the follwing syntax

FROM d1 TO d2 DO y(t)=f(y(t-1),e(t),e(t+1),e(t-1))

where d1 and d2 are @dates objects (d1<d2), y is a @dseries object
defined over d1-1:d2, and e is a @dseries object defined over
d1-1:d2+1.