Standardize the freud API

[Bradley Dice]

From roadmap planning session with @vramasub and @bdice. To be assigned after #176 is closed.

Standardize the freud API

Once #176 (doc cleaning) is done, we should identify what we want APIs to look like for everything in freud.

1. Review existing APIs
2. Determine a standard for all modules to follow, with minimal exceptions
3. Create a list of cases where the standard is not currently followed

The behavior of compute/accumulate/property getters is one notable case that should be standardized.

For methods that we want to remove (e.g. getRDF, which should be replaced by a property), we will remove that method from the documentation and add deprecation warnings for version 2.0.

For any class/function where the signature has to change, we will make use of *args, **kwargs to take variable APIs and then dynamically resolve them. Deprecation warnings will be issued wherever appropriate.