Deal with brekpoints in the periodic case.

In the following, what I mean by default discretization is the DISCRETIZATION field of CHEBOPPREF.
By default, it is CHEBCOLLOC2. But the user can change it typing
cheboppref.setDefaults('discretization', @anyDiscretizationTheUserWantsToUse).

Before, if using the 'periodic' tag, the discretization used was FOURCOLLOC, unless
the default discretization in CHEBOPPREF was ULTRAS (in this case ULTRAS was used).
Moreover, if the user was using a PREF object to solve the bvp, that is u = solvebvp(L, f, pref),
PREF.DISCRETIZATION was used.

Now, if using 'periodic', the discretization used is FOURCOLLOC, unless the default discretization is
ULTRAS (in this case ULTRAS is used), or if there is a breakpoint (in this case the default discretization is used, whatever it is).
It the user is using a PREF object, PREF.DISCRETIZATION is used.
If there is a breakpoint and the discretization used is FOURCOLLOC (either because FOURCOLLOC is the default discretization, or because the users passed a PREF object with FOURCOLLOC in PREF.DISCRETIZATION), we
throw an error.
Correction suggested by Asgeir.
Change the way we modify the preferences when using periodic boundary conditions.
Adding documentation.
Merge branch 'fix-breakpoints-periodic' into development