use a local context cache instead of a global one

[mikemhenry]

Description

Instead of using the global context cache (which is the default) create a instance of the context cache and use that for all the mcmc move types. I also updated an example code snippet as well.

Motivation and context

See this issue #613 (comment)

How has this been tested?

Tested on CI, issue linked above also shows that this fix works for large systems.

Change log

Use an instance of ContextCache instead of the default global instance.

[codecov]

Codecov Report

Merging #849 (7f682cd) into master (ba3e04e) will increase coverage by 0.00%.
The diff coverage is 66.66%.

[ijpulidos]

Yes, this works! Great!. We just have to keep an eye on issues with different (larger?) systems, but I believe an issue is already created for that in the openmmtools repo.