Cleaning up platform and context usage in geometry engine

[ijpulidos]

Description

By default the CUDA platform was hardcoded in the geometry engine. Generating contexts that were not handled correctly. This PR is aimed towards making CPU to be the default platform for the geometry engine and cleans explicitly created contexts for a better memory management.

Motivation and context

Resolves #1085

How has this been tested?

Change log

``CUDA`` platform was hardcoded in geometry engine, generating performance issues by not clearing openmm contexts correctly. Fixed by defaulting to using the faster ``CPU`` platform (for the geometry engine) and explicitly deleting context variables after they are used.

[codecov]

Codecov Report

Merging #1091 (9fad690) into main (37da4fb) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

[mikemhenry]

Maybe just a slight change in the change log (to make it clear that we are only setting the platform for the geometry engine:
"CUDA platform was hardcoded in geometry engine, generating performance issues by not clearing openmm contexts correctly. Fixed by defaulting to using the faster CPU platform (for the geometry engine) and explicitly deleting context variables after they are used."

[mikemhenry]

Not sure why RTD isn't working, builds have been working just fine: https://readthedocs.org/projects/perses/builds/
I will merge this in any way and we can check on RTD later.

[zhang-ivy]

Instead of changing the DEFAULT_PLATFORM in tests/utils.py, we want to change this line to

added_energy_components = compute_potential_components(mod_context, platform=platform)