Update pose display hook to evaluate dssp without modifying pose.
Convert PDBEmbed to operate on list of repr selections.

Updating PDBEmbed to operate on list of repr selections, instead of
explict repr strings. First step towards json-declarative representation
model. Updating GLMol to support additional specifier syntax.
Update pose_display to pass repr entries to underlying PDBEmbed.
Handle None is repr_entries.
Update GLmol with bond detection patch.
Add support for `zoom` representation entries.