Also removed automatic installation of dependencies (done in gitlab-ci
pipeline instead).

... returned when one tries to instantiate a dates object with an
object which cannot be interpreted as a date.

If non singleton objects have different number of elements, return
false instead of raising an error message.

Also adapted runalltests script (saves an empty file called failed iff
one of the unit tests fails).

In the base directory:

~$ make check-octave

will trigger the testsuite with Octave, and

~$ make check-octave

triggers the same testsuite with Matlab.

With Octave `which('dates')` returns nothing (Octave claims that `dates`
is a built in function).

Install m-unit-tests module from Github/master if needed.

As in commit 6beaf7b0 (for the sort method).

As in commit 6beaf7b0 (for the sort method).

As in commit 6beaf7b0 (for the sort method).

As in commit 6beaf7b0 (for the sort method).

... and added a new method.

sort_ sorts a dates object in place (without a copy).
sort  first make a copy and then sorts the copy.

The first routine, with in place modification, is more efficient because
we do not have to create a new object.

Example
=======

If o is a dates object with more than one element, then

o.sort()

returns a sorted version of o without modifying o, while

o.sort_()

sorts the elements in o.

Also (re)activated the unit tests.