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.