All dseries syntaxes now work under Octave, except ts{'variable'}(range) = value.

This member keeps track of the transformations and operations on the
variables. The name of the variables is now constant.

There is a bug in Octaveo's class implementation (see https://savannah.gnu.org/bugs/?46571).

Also deactivated some of the tests.

 + Code factorization.
 + Allows regular expressions in subsasgn method.

It is not possible to instantiate an empty dseries object with a time span.