Extend Topology.from_openmm() to include `positions` argument

[jchodera]

Is your feature request related to a problem? Please describe.
Currently, the idiom for creating a Topology object from an OpenMM setup pipeline (e.g. openmm.app.Modeller or pdbfixer) is

openff_topology = openff.toolkit.topology.Topology.from_openmm(modeller.topology, unique_molecules=molecules)
import openff.units.openmm
openff_topology.set_positions( openff.units.openmm.from_openmm(modeller.positions) )

Describe the solution you'd like
Adding an optional argument positions to Topology.from_topology() could simplify this to

openff_topology = openff.toolkit.topology.Topology.from_openmm(modeller.topology, positions=modeller.positions, unique_molecules=molecules)

with automatic conversion of the OpenMM positions as needed.

Describe alternatives you've considered
The verbose version is workable, but clunky and confusing.

[j-wags]

Good idea!