SMILES matching on UA need a better warning or different functionality

[NoahMontrose]

Using SMARTS matching on an United Atom system without the add_hydrogens flag gives a confusing result.
This works

pps = mb.load("c1ccc(S)cc1", smiles=True)
pps.save("pps_box_H.gsd", overwrite=True)
gsdfile = 'pps_box_H.gsd'
system = grits.CG_System(
            gsdfile,
            beads={"_A": "c1ccc(S)cc1"},
        )
system._compounds[0].visualize(show_atomistic = True)

This gives a warning

pps = mb.load("c1ccc(S)cc1", smiles=True)
pps.remove(pps.particles_by_element('H'))
pps.save("pps_box_noH.gsd", overwrite=True)
gsdfile = 'pps_box_noH.gsd'
system = grits.CG_System(
            gsdfile,
            beads={"_A": "c1ccc(S)cc1"},
            #add_hydrogens=True
        )
system._compounds[0].visualize(show_atomistic = True)

/Users/noah/miniconda3/envs/grits/lib/python3.8/site-packages/grits/coarsegrain.py:139: UserWarning: c1ccc(S)cc1 not found in compound!
  warn(f"{smart_str} not found in compound!")
/Users/noah/miniconda3/envs/grits/lib/python3.8/site-packages/grits/coarsegrain.py:169: UserWarning: Some atoms have been left out of coarse-graining!
  warn("Some atoms have been left out of coarse-graining!")

This warning is confusing when reading from a United Atom gsd and using the corresponding SMARTS string. It's not obvious that the hydrogens need to be added back in this case.

We should either add to the warning specifying that we might need the add_hydrogens=True flag or make the behavior automatically add hydrogens back to United Atom systems.