Return the number of slices processed

The function zappend.api.zappend() now returns the number of slices processed.

Closes #93

Checklist (strike out non-applicable):

[x] Changes documented in CHANGES.md
[x] Related issue exists and is referred to in the PR description and CHANGES.md
[x] Added docstrings and API docs for any new/modified user-facing classes and functions
[ ] ~Changes/features documented in docs/*~
[x] Unit-tests adapted/added for changes/features
[x] Test coverage remains or increases (target 100%)

bcdev / zappend