Rename Power_m_p to StaticPower, which is a bit more telling, and conforms better to the Dune naming scheme

[[Imported from SVN: r7037]]

Taken from hcube.cc in dune-grid.

We may want to unify the naming eventually.

[[Imported from SVN: r7033]]

There are lots of power implementations all over our code.
This is the first step of some cleanup.  It moves the class Power_m_p
from misc.hh (like who's ever gonna find it there...) to a separate
header power.hh.  Other implementations will be collected there as
well, if appropriate.

With the patch, including misc.hh will issue a cpp warning that people
who include misc.hh only for Power_m_p should now include power.hh
instead.  This is not great, because it also affects people who include
misc.hh for something else.  Bear with this for a while; I couldn't
think of a better solution.

[[Imported from SVN: r7032]]