Dump a grid to a pdb file at a given iso value

Based on a script from mdpocket/fpocket (available open source). 

Hi Oli, I'm converting your grid to a different representation to mimic the original extractISOPdb.py script. Is there a less crummy way to do the conversion? Maybe there is a better way altogether to express what extractISOPdb is doing in a the framework of griddatafromats.