Changed subsref overloaded method (dynDate): added a new interface to populate empty dynDate objects with specified frequency. Added unitary test.

For instance if we instantiate an empty dynDate object for quaterly dates:

qq = dynDate('Q');

Then we can populate this object as follows (2009 second quarter):

d1 = qq(2009,2);

Allow to populate an empty dynDate object or update a dynDate object. For instance, if we instantiate
the dynDate class as follows:

a = dynDate()

a is an empty dynDate object. We can "populate" this object as follows

a('1938Q4')

Note however that a ins not modified in place (due to a limitation of Octave's class design), a copy
has to be made.

Changed name of dynDates class to dynDate. dynDates will be used latter for irregular and unsorted sequence of dates.