v2: extract split/drop-cycle helpers from cellreader (V2-09 follow-up)

[jepegit]

V2-09 continuation (epic #402; follow-up from #509). Target branch: master.

Extract the split/drop-cycle helpers from cellreader.py (twice-flagged
# TODO: consider moving splitting etc outside of CellpyCell):
mod_raw_split_cycle, split, drop_from, drop_to, from_cycle,
to_cycle, drop_edges, split_many, with_cycles.

Follow the #509 pattern (cellpy/readers/capacity_curves.py): verbatim move
into e.g. cellpy/readers/slicing.py, instance-first functions, thin
delegates, behaviour unchanged. Check existing test coverage of
split/split_many/with_cycles before moving; add pins where thin.

[jepegit]

Implemented in #527 (merged into the #524–#528 stack; landed on master via #525, commit 9f81662). Split/drop-cycle helpers moved verbatim to cellpy/readers/slicing.py with thin delegates; coverage pins added for from_cycle/to_cycle/drop_edges/split_many(list)/with_cycles. Closing manually — the PR merged into a stack branch, so auto-close never fired.