Consider making one end of the slice bounds optional

[achaikou]

Now one has to provide both lower and upper bounds.
It might be inconvenient if users would often like to limit the request only from one side. It might be better having the non-supplied bound automatically expand to the whole of the cube.

However PO wasn't sure that is really something users really need.
So this task first implies finding out if this feature is wanted and implementing if it is.