Add state msgs

[matthew-reynolds]

Pull Request

Closes #29 Closes #80

Adds messages that will be used throughout the stack for tracking the state of the field and of entities.

Contribution Checklist

• [x] I have read the contributing guide.
• [x] I have referenced relevant issues.
• [x] I have labeled accordingly.
• [x] I have detailed my changes as much as possible.

Change Checklist

• [ ] I have added tests.
• [x] I have added necessary documentation.
• [x] I have auto formatted using clang-format or yapf.
• [ ] I have tested my changes on real hardware.

[matthew-reynolds]

Why will float64s be used as inputs to the decay function, do we really need a full float64 for the confidences?

[wmmc88]

Upon receiving new information about state, the new confidence value will be a function of the old value, new value and confidence in the new value

[wmmc88]

Actually on second thought that has nothing to do with the msg and will be handled internally within the node itself