Power pot templates

Add template expansions of power function for integer and half-integer exponents.
Add integer and half-integer versions of inverse power potential.
Add Hook and Hertz specializations to Cython part.

The template meta power functions are maybe faster than the normal pow.
On the c++ side the usage comfort is more or less OK.
On the python side, as long as we mainly want Hook and Hertz, it should also be OK. 
you could also put in a static assert here that N is divisible by 2