Consider making ElectrostaticsCollection.invalidate_cache

[mattwthompson]

          Am I right in thinking that this has implications for modifying an `ElectrostaticsCollection` after the charges have been cached? If so, since we demonstrate modifying collections in the examples, we should probably add some sort of `invalidate_cache()` method and document this behavior both in the collection docstring and in the relevant examples. Modifying a collection according to the example and having that modification only sometimes reflected in outputs would be quite frustrating.

Originally posted by @Yoshanuikabundi in #1115 (review)