Add `vdWHandler.long_range_dispersion`

[mattwthompson]

Pending https://github.com/openforcefield/standards/pull/40

• [ ] Tag issue being addressed
• [x] Add tests
• [ ] Update docstrings/documentation, if applicable
• [x] Lint codebase
• [ ] Update changelog

[codecov[bot]]

Codecov Report

Merging #1351 (cc995d9) into main (a38d062) will decrease coverage by 0.20%. The diff coverage is 80.00%.

[mattwthompson]

Here's a look as some behaviors in this implementation:

>>> from openff.toolkit.typing.engines.smirnoff.parameters import vdWHandler
>>> from openff.toolkit import ForceField
>>> ForceField("openff-2.0.0.offxml")['vdW'].long_range_dispersion
'isotropic'
>>> vdWHandler(version=0.3).long_range_dispersion
'isotropic'
>>> vdWHandler(version=0.3).version
<Version('0.4')>
>>> vdWHandler(version=0.4).long_range_dispersion
'isotropic'
>>> vdWHandler(version=0.4, long_range_dispersion="none").long_range_dispersion
'none'
>>> handler = vdWHandler(version=0.4)
>>> handler.long_range_dispersion
'isotropic'
>>> handler.long_range_dispersion = "none"
>>> handler.long_range_dispersion
'none'

[mattwthompson]

Closing as this has stalled, but it is linked in the EP for posterity.