add element to slice dialog to have the option to select no slices at all

Otherwise, if at least one slice exists, you have to select one of them
Update selectedSlice entry after choose a new one
Enrich assets/cubeWithMaterializedSlices.ttl with one more slice

And fix base uri
Fix URI's in assets/cubeWithMaterializedSlices.ttl
Fix wrong attribute/measure relations in assets/cubeWithMaterializedSlices.ttl
Make slice selection fully runable (could by fragile sometimes)