Unify access to centroids etc. in CpGrid

[atgeirr]

As just merged, #261 adds new ("Ecl") family of methods to access geometric measures computed with a different paradigm and goal from the existing methods. A problem raised in that discussion was that this is not accessible using the regular Dune grid interface, and that users are confronted with the choice of geometry at each grid call site.

A solution has been proposed to handle this by rather passing a flag upon construction to choose "method family" for geometry, storing and accessing the results in the same (standard) way afterwards.