Adding abstract GridDF

[adamamer20]

This is the last abstract class before the implementation of the concrete Grid classes.

• Normalization was added to dataframe mixin.
• One unique class for all neighborhood types. The type is specified at the start (easier to work with than having multiple classes).
• The neighbors are found by considering the agents in the neighborhood, not the distances between agents. Need to evaluate which is faster and whether to create a conditional implementation (eg distances with a few agents and neighborhood with many agents)

[rht]

Other than the comments, everything else LGTM.