New has_variation(id) method 

This method is used for querying if a unit has a certain subtype.
If the unit is a variation itself it checks the siblings.
SUF: New filter "has_variation"; renamed "variation_type"->"variation"

The new has_variation SUF attribute filters if a unit has a variation
child type with the given id. If the unit is a variation itself the
siblings are considered.
Replace all kinds of the constant pi representations.

Replaces every appearance of the circle constant pi I could find with
the representation from boost/math/constants/constants.hpp.