Closes #21.

It is possible to rename the dseries class. For instance, the
following is a valid syntax:

>> tseries = dseries();
>> tseries(ones(3,2))

ans is a dseries object:

   | Variable_1 | Variable_2
1Y | 1          | 1
2Y | 1          | 1
3Y | 1          | 1

It is also allowed to define a default starting period for the data passed
to the renamed dseries class/object:

>> tseries = dseries('1938M11')
>> tseries(ones(83,2))

ans is a dseries object:

        | Variable_1 | Variable_2
1938M11 | 1          | 1
1938M12 | 1          | 1
1939M1  | 1          | 1
1939M2  | 1          | 1
1939M3  | 1          | 1
1939M4  | 1          | 1
1939M5  | 1          | 1
1939M6  | 1          | 1
1939M7  | 1          | 1
1939M8  | 1          | 1
        |            |
1944M11 | 1          | 1
1944M12 | 1          | 1
1945M1  | 1          | 1
1945M2  | 1          | 1
1945M3  | 1          | 1
1945M4  | 1          | 1
1945M5  | 1          | 1
1945M6  | 1          | 1
1945M7  | 1          | 1
1945M8  | 1          | 1
1945M9  | 1          | 1