Class Fragment

@param length: length of the fragment
@type length: int

@param fid: id for the fragment
@type fid: int

Overrides: object.__init__

@return: length of fragment
@rtype: int

Returns <Fragment length=L id=ID> where L=length of fragment
and ID the identifier (rank in the library).

Overrides: object.__repr__

Return rmsd between two fragments.

Example:
    >>> rmsd=fragment1-fragment2

@return: rmsd between fragments
@rtype: float

@param resname: residue name (eg. GLY).
@type resname: string

@param ca_coord: the c-alpha coorinates of the residues
@type ca_coord: Numeric array with length 3

@return: the CA coords in the fragment
@rtype: Numeric (Nx3) array

@return: id for the fragment
@rtype: int

@return: the residue names
@rtype: [string, string,...]