Need AbstractLatticeCluster type so can write generic code for lattice clusters

[BrianGun]

Fixes #301

Added AbstractLatticeCluster type and more documentation explaining the interface associated with the type.

Pull Request Template

Description

Want to write generic functions that accept any type of lattic cluster object but currently there isn't an abstract type that all lattice cluster concrete types inherit from. Added this type and updated documentation to describe the interface of AbstractLatticeCluster.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x ] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

added test to verify that declaring function arguments to be of type AbstractLatticeCluster works.

Checklist:

• [x ] My code follows the style guidelines of this project
• [ x] I have performed a self-review of my own code
• [ x] I have commented my code, particularly in hard-to-understand areas
• [ x] I have made corresponding changes to the documentation
• [ x] My changes generate no new warnings
• [ x] I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• [ x] I have checked my code and corrected any misspellings