Class ExposureCN

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

Overrides: object.__init__