Fix return type of integration element

Back then when proposing the unit safe interface for the grid, I was unsure whether the integration element should return in global coordinates on in local coordinates. Today, after re-implementing a unit-safe wrapper and used it for a FEM calculation, I realized that it should indeed return in global coordinates. In other words, it should return Volume instead ctype. Furthermore, a dimension analysis of the integration element formula yields the volume units in global coordinates. In other words, \Upsilon\left(\mu(x)\right) = \Upsilon(W)^d , where \Upsilon is the unit signature operator and \Upsilon(W) the unit of the global coordinate type.

This MR fixes the issue in dune-grid and changes integrationElement to return Volume instead ctype.