Audit and Pin down return types of methods across chaco

**Problem Description**
See comment: https://github.com/enthought/chaco/pull/636#issuecomment-816879211

There are many places in chaco where we aren't super consistent about the type being returned by specific methods and this has led to a variety of bugs / issues such as:
#255, #272, #289, #550, (possibly) #542

**Expected behavior:**

The return types should be consistent as defined on the relevant interface or base class to prevent bugs in strange edge cases.