Simplify using custom slice sources

[forman]

Is your feature request related to a problem? Please describe.

Using custom slice sources can be simplified by not forcing slice sources to return a SliceSource instance.
Instead, slice source functions should be allowed to return a SliceObj.

Describe the solution you'd like

As described above.

• Rewrite tests, add new, implement changes
• Rename argument slice_obj into slice_item to underline it is one of the elements of the slices iterator passed to zappend
• Update (simplify) user guide, especially confusing section about slice sources and slice factories