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.
Fixed case sensitivity issue with respect to TO and DO keywords.
Factorized code for the displayed error messages.
Fixed typo in comments.
Removed the loop for building the recursive expression to be evaluated.
Added comments.
Check that the initial and terminal dates are in the ranges of the
dseries objects involved in the recursive expression.
Added lead/lag incidence table. Also check that the ranges of the
dseries objects appearing in the recursive expression are compatible
with the range of the loop.
Added tests on the recursive expression to be evaluated (the endogeous variable
canot depend on its current level or its future level).
Code optimization. Removed manipulations of dseries objects in the
main loop.
Save empty dates and dseries objects on disk (in dynareroot), to reduce the time needed to instantiate new objects.
Exclude from versioning the mat files generated for the initialization of dseries and dates objects.
Save empty dates and dseries objects in memory (base workspace) instead of disk (faster). Added an initialization mode for dates and dseries constructor. The following commands:

dates('initialization');
dseries('initialization');

create variables emptydatesobject and emptydseriesobject in base workspace.
Improve speed of @dates/colon method.
Improve speed of comparison methods (lt, gt, le and ge) for @dates class.
Improve speed of @dates/colon method (added specialized version of add_periods_to_array_of_dates -> add_periods_to_date).
Removed call to ismember. Test if to_id or do_id are empty instead.
Another speed improvement for @dates/colon method (vectorized code, removed loop around add_periods_to_date routine).
Bug fix. Wrong number of lines for time member if n<freq.
Various optimizations and cosmetic changes.