Merge both new swapping methods together

[ajfriedman22]

Description

Provide a brief description of the PR's purpose here.

Type of Change

• Bug fix
• New feature
• Documentation update
• Unit test and CI update
• Other

Todos

Notable points that this PR has either accomplished or will accomplish.

• Add al alternate swapping method where the number of molecules of interest changes rather than changing a portion of a molecule between replicas

Checklist

Starting from PR #62, external PRs must keep the following checklist in the PR.

• I have read the contributing guidelines.
• My code follows the project’s coding style and is free from linting errors examined by flake8.
• My code comes with docstrings that follow the NumPy docstrings standard.
• I have updated the documentation in the docs directory if my changes affect the project's functionality or usage.
• I have added new tests for new features or changes to existing features. The code coverage is at least 90%.
• My code passes all unit tests and CI tests on CircleCI.