Initialize.

A residue’s exposure is defined as the number of CA atoms around that residues CA atom. A dictionary is returned that uses a L{Residue} object as key, and the residue exposure as corresponding value.

:param model: the model that contains the residues :type model: L{Model}

:param radius: radius of the sphere (centred at the CA atom) :type radius: float

:param offset: number of flanking residues that are ignored in

           the calculation of the number of neighbors

:type offset: int