[Biopython] PDB occupancy behavior

[Lenna Peterson]

Hi all,

I just submitted a pull request I'd like wider feedback on.

https://github.com/biopython/biopython/pull/207

In summary, I am using software-produced PDB files that simply stop after
the coordinate data, so occupancy data is missing. Currently, the Biopython
PDBParser sets missing or blank occupancy to 0.0. I am suggesting changing
this to 1.0.

I would like to see if anyone knows of situations in which this would be a
bad idea.

Cheers,

Lenna