Changed behaviour of @dynSeries/size.

If called with a second input argument (dim), size returns the number of observations if dim==1 or
the number of variables if dim==2. Without this second argument, size returns the number of
observations and the number of variables.
Give dot access to the @dynSeries/size function.
Added unitary test for @dynSeries/size.
Added iscellofchar function.

Test if all the elements of a two dimensional cell array are strings.
Added the possibility to extract variables from dynSeries object using indices instead of variable names.

If ts is a dynSeries object containing variables A1, A2, A3, A4, A5, then the following syntaxes are equivalent:

ts{[1, 2, 5]}

ts{'A1','A2','A5'}

and will create a new dynSeries object made of variables A1, A2 and A5.
Added unitary test in dynSeries/subsref.