for dseries (see 48115cb8, which does not work with Octave) or a
method (see ef923e41, which does not work with all supported Matlab's
versions) to initialize the dseries class.

Fixed dependency on m-unit-tests toolbox.

Takes into account the change for the dseries initialization routine.

To initialize the toolbox:

 - Add the `src` folder in the path
 - run `dseries().initialize()`

This is an Octave compatibily workaround, Octrave cannot handle
simultaneously +folder (namespace) and @folder (class).

Using the old HTTP URL makes test fail.

For a zero, it returns -Inf.
For a negative number, it returns NaN.

This is not exactly the same behavior as MATLAB's log function, because we want
to enforce type stability (i.e. never return a complex).

'A', 'Q', '1900Q', 'Q1', 'M', '1900M', 'M1','W', '1900W' and 'W1'
should not be interpreted as dates.

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