Tentative implementation of periodic variables

Implementation of periodicity as we discussed at the meeting.
In the constructor of a collective variable or function is it possible
to declare, for each value, if it is periodic or not. E.g., torsions
will be periodic, distances not. The difference between two values
(hypothetically, a stored value and the present one) can than
be computed automatically using a Value::difference() method.
If difference() is called for undeclared values, an error is raised.
Thus, functions of unpredictable periodicity (e.g. MATHEVAL) should
allow the user to specify the periodicity (or the non-periodicity)
in the input line.
See how I implemented the setting of periodicity in the available
Colvars. The syntax is a bit baroque (I will probably simplify it),
but it seems to work.

I also added a check on the fact that two labels can not be equal.