B4: collectors — plotting handover

[jepegit]

Epic B — Collectors · Phase 3 — plotting handover (with plotting already in 2.0). Plan §4 Phase 3.

Acceptance

• Collection.plot() and BatchCollector.plot() wired to cellpy.plotting.
• Save bundle (data + figure + serial-numbered filenames) goes through plotting.figures.
• Old collectors' plotter functions/fig-IO/templates deletion is scheduled in E4 (breaking).

Depends on B3 + A8. Part of #696.