Addition of a private member to store data

[sauk2]

Desired behavior

Currently, the BasicSystem and FullSystem plugins do not have a private member to store data either extracted from the SDF or created internally during plugin operation.

Implementation suggestion

• Forward declare a class and create a unique pointer to the same in the header
• Define an implementation of the class in the cpp
• Add lines in Configure to get values from the SDF and assign them to the private member
• To show functionality, add parameter tags in the XML of the plugin in the world SDF

PS. If this is a valid request and no one has started working on this, I have an implementation in my fork for which I can generate a PR.