PI is no longer internal as is used all over the place and was kind of
hard to find. More centrally, the constants have been refactored to
separate the unitless ones (PI, NA), which are now public, from the
ones with units which will remain internal until something nice
happens to units (that will probably wait a year or so until we adopt
Boost 1.36 or whichever version first introduced the Boost.Units
library).