Treat geometries by its lower left and upper right corner

Yasp treated geometries by taking the midpoint and size information,
although it stored the information as lower left and upper right
corner. AxisAlignedCubeGeometry also asks for the latter. With this
patch, this transformation is omitted. Additionally, YgridComponent::
Iterator doesnt store positions anymore but instead asks the coordinate
container directly.
[doc] Document that YaspGeometry and AxisAlignedCubeGeometry are the same now.