write.pdb does not include mandatory fields

The PDB format specifies certain fields as mandatory. Currently, write.pdb just writes atoms. Expected behavior would be to write a valid PDB file. Alternatively, including the minimal fields necessary for parsing by other common tools would be a good first step.

If full-format output is attempted, work on read.pdb would probably be required, since it does not parse most of these fields. Alternatively, dummy values could be written.