add constants

[gpeterhoff]

Can you please add 2 constants to boost::math::constants:
root_five = sqrt(5)
psi = 1 - phi
These are needed e.g. for the calculation of Fibonacci numbers.

thx
Gero

[mborland]

The value of psi is available as reciprocal_fibonacci. I can add root_five. We also have <boost/math/special_functions/fibonacci.hpp> if that's what you need.

[gpeterhoff]

I meant this psi = -0.618033988749...