Add numeric chain index and TER parsing.

Adding atom property `chaini` containing a numeric chain identifier,
auto-incremented as chains are read. Adding support for TER entries in
pdb parser. Updating code to use chaini, instead of chain name, to
determine residue connectivity.
Update selection parsing to support modifiers and multiple properies.

Updating parseSelectionToAtomList to support `chaini`, `atomi`, and
`resi` based selection ranges. Adding `bb` and `heavy` modifiers to
selection backbone atoms and non-hydrogen atoms.
Add `all` selection mode and initial `elem C` modifier.
Update bond detection to fix cross-chain, non-disulfide bonds.
Add `zoom` representation entry.