Avoid duplicate API documentation entries.

Currently, classes and functions that are implemented in subpackages and imported into higher level namespaces are documented multiple times. For instance, Ball is documented in full at picos.expressions.set_ball.Ball, picos.expressions.Ball and picos.Ball. This increases documentation compile time, renders the documentation pages for the higher level packages very long and turns the PDF documentation into a 600 page monster.

Ideally, the entries for picos.Ball and picos.expressions.Ball should only include a link to the documentation of picos.expressions.set_ball.Ball.