cannot inherit from annotation _Feature

[CHANGED] adding a test to expose problems with inheriting from
    cogent.core.annotation._Feature. Many methods are hard-wired to return
    a _Feature class instance. This patch just introduces a test demonstrating
    this issue. A fix is in the next patch.
support subclassing of _Feature

[FIXED] patch that improves support for inheriting from _Feature. This patch
    allows the test introduced in the previous patch to pass.