Always read topology types from GSD file.

[joaander]

Description

Read topology types from GSD files, even when there are no groups in that topology entry.

Motivation and context

The type names in the GSD file should be preserved.

Resolves #1722

How has this been tested?

This test script passes:

import hoomd
import gsd.hoomd

L = 5
frame = gsd.hoomd.Frame()
frame.particles.N = 1
frame.particles.position = [0, 0, 0]
frame.particles.typeid = [0]
frame.configuration.box = [L, L, L, 0, 0, 0]
frame.particles.types = ['A']

frame.bonds.types = ['A-A']
frame.dihedrals.types = ['dihedralA']
frame.impropers.types = ['improperA']
frame.angles.types = ['anglesA']
frame.pairs.types = ['pairA']

with gsd.hoomd.open(name='init.gsd', mode='w') as f:
    f.append(frame)

sim = hoomd.Simulation(device=hoomd.device.CPU(), seed=1)

sim.create_state_from_gsd(filename='init.gsd')

print(sim.state.bond_types)
print(sim.state.angle_types)
print(sim.state.dihedral_types)
print(sim.state.improper_types)
print(sim.state.special_pair_types)

Change log

Fixed:

* ``create_state_from_gsd`` reads bond/angle/dihedral/improper/pair types when there are no
  corresponding groups (`#1694 <https://github.com/glotzerlab/hoomd-blue/pull/1694>`__).

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.