Refined mesh surface extraction

Added an angle-parameter to extract surfaces more precisely.
Before: All surfaces were extracted where the normal didn't deviate from the given vector by more than 90 degrees.
Now: All surfaces are extracted where the normal doesn't deviate from the given vector by more than the given angle, with 0 < angle < 90 degrees.
Why not use pi from boost directly? But then I would directly use `boost::math::constants::radians`: http://www.boost.org/doc/libs/1_56_0/libs/math/doc/html/math_toolkit/constants.html