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.

to retrieve the label and the set of dependencies

removed unused method and other clenup

Small inconsistency in usage of setValue removed

I removed some unused methods to manipulate atoms

I removed the claim that I am the only author and cleaned a bit
the introduction

This is the draft version that I prepared and that we will use as
a starting point. Still it is a bit dirty, and I will try to improve
documentation and delete unused routines