v2: slim CellpyCell, public API cleanup, exclude_step_types (V2-09/10/12)

[jepegit]

v2 Phase 3 — thin orchestration and API (epic #402: themes V2-09, V2-10,
V2-12). Target branch: master. Depends on: Phase 1 data model stable.

Work

• V2-09 Slim CellpyCell (cellreader.py): extract load/save/merge/plot
  utilities (the # TODO clusters); dependency injection for testability.
  Behaviour unchanged, measurable line reduction.
• V2-10 Public API cleanup (cellpy/__init__.py, docs): resolve the
  # TODO: (v2.0) shortcuts; explicit import paths (cellpy.get, session
  module). Deprecation warnings in v2.0; removal plan documented.
• V2-12 Expose the native exclude_step_types from core (core Pin scipy to latest version 0.19.0 #54) on
  make_summary, replacing the removed selector-based exclusion; must match
  legacy exclusion behaviour on fixtures.

Refs

Epic #402 / cellpy-v2-epic.md Phase 3.