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.

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.

In addition to the already implemented selection mechanism using matlab's regular expressions,
we allow the following syntax:

a = ts{'GDP_@US,JP,FR@'};

to extract variables GDP_US, GDP_JP and GDP_FR from dynSeries object ts. If one of these variables
does not exist, an error message is issued.

Names for the results of basic operations on dynSeries objects was '--NA--'. The name of the result
variables now keep track of the operations. For instance, if we have two dynSeries objects, ts1 and
ts2, respectively holding two variables 'A1' and 'B1', then:

 - the name of the variable in ts3=ts1+ts2 is 'plus(A1,B1)'
 - the name of the variable in ts3=ts1-ts2 is 'minus(A1,B1)'
 - the name of the variable in ts3=ts1*ts2 is 'multiply(A1,B1)'
 - the name of the variable in ts3=ts1/ts2 is 'divide(A1,B1)'
 - the tex name of the variable in ts3=ts1+ts2 is '(A1+B1)'
 - the tex name of the variable in ts3=ts1-ts2 is '(A1-B1)'
 - the tex name of the variable in ts3=ts1*ts2 is '(A1*B1)'
 - the tex name of the variable in ts3=ts1/ts2 is '(A1/B1)'

Returns zero if the input argument is a well defined dynSeries object.

 + Added the possibility to rename the assigned variables.
 + Added unitary tests.

Issue an error if two dynSeries objects with common variable names are concatenated. Added a unitary test.

Removes a variable form a dynSeries object.

If the names of the variables are not provided when instantiating a dynSeries object
then the default name is of the form Variable_i (i=1,...,N, where N is the number of
variables in the dataset).

+ Prefix variable names with QGROWTH_
+ Prefix tex variable names with \delta_i with i=nothing or 3 depending on the frequency.

 + Prefix variable names with YGROWTH_
 + Prefix tex variable names with \delta_i with i=nothing, 4, 12  or 52 depending on the frequency.

+ Prefix variable names with QDIFF_
+ Prefix tex variable names with \Delta_i with i=nothing or 3 depending on the frequency.