The geometry implementation used an extra call to the `corners()`
method to get the number of corners before.  As the compiler could not
be sure this is the same number as `Corner_Coordinates()` uses, there
were several compiler warnings (from gcc 7.2) of the form

```
dune-uggrid/gm/evm.h: In member function ‘[...] Dune::UGGridGeometry<[...]>::volume() const [...]’:
dune-uggrid/gm/evm.h:88:67: warning: ‘*((void*)& cornerCoords +24)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
dune-grid/dune/grid/uggrid/uggridgeometry.hh:96:16: note: ‘*((void*)& cornerCoords +24)’ was declared here
```

With this change the wrapper function returns the number of corners
just as the dune-uggrid macro does.  Using this value instead of the
`corners()` call silences the compiler warnings.