Move method 'power' to namespace Dune::Common
The recent commit b6eca40e introduced a method called 'power' in the namespace Dune. Rather quickly, this lead to name clashes with the method of the same name in dune-functions. The method there is contained in the namespace Dune::Functions::BasisFactory, which many people import completely.
As a remedy, this patch moves the 'power' method into the new namespace Dune::Common.
I now that there has been a democratic decision not to have module-specific namespaces for the core modules, but given that power
is a very generic name, and that the method was introduced only very recently, I plead for an exception.