Remove OpenMM dependency from submodules

[bbye98]

In the algorithm and analysis submodules, the unit conversions and handling should be transitioned from openmm.unit to pint. However, functions and methods that accept openmm.unit.Quantity objects should retain their type hints (change unit.Quantity to "openmm.unit.Quantity") and unit conversions (change isinstance(data, unit.Quantity) to type(data).__name__ == "Quantity").

In the openmm submodules, no changes are necessary since openmm is a required dependency.

[bbye98]

The pint.UnitRegistry() should probably be defined either in the base __init__.py file or the __init__.py file for the analysis submodule.

[pw0908]

For my own reference:

• Fix openmm.app dependency in topology.py

[bbye98]

Consolidated into issue #4 as part of the MDHelper 1.1.0 release.