Legacy support of PDP/PQR as we move to CIF

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As we move to supporting CIF for input and output, we need to map the PDB records to PQR and CIF output.

This could be done in a few different ways.

For reading PDB and writing PDB, PQR, and CIF formats:

1. The PDB classes could stay the way it is and provide additional methods:

• to_pdb - provide a string representation of the record in PDB format
• to_pqr - provide a string representation of the record in PDB format plus charge and radii fields
• to_cif - provide a string representation of the record

For reading CIF and writing PDB, PQR, and CIF formats:

1. The CIF containers would have to be "adapted" to PDB classes