Add DGP functions to prep.py for all supported DiD designs

Consolidate Data Generating Process functions from tutorials and tests
into diff_diff/prep.py as reusable library utilities:

- generate_staggered_data(): Staggered adoption DiD (CallawaySantAnna, SunAbraham)
- generate_factor_data(): Factor model data (TROP, SyntheticDiD)
- generate_ddd_data(): Triple Difference (DDD) designs
- generate_panel_data(): Panel data with optional parallel trends violations
- generate_event_study_data(): Event study with simultaneous treatment

Changes:
- Add 5 new DGP functions to diff_diff/prep.py with full documentation
- Export new functions from diff_diff/__init__.py
- Add 33 tests covering all new functions in tests/test_prep.py
- Update test files to use library functions where compatible
- Update tutorials 02, 04, 07, 08, 10 to import from library
- Fix pre-existing API bug in tutorial 07 (show_mdv -> mdv parameter)

Users can now generate synthetic data via:
  from diff_diff import generate_staggered_data, generate_factor_data, ...

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: igerber

diff_diff/__init__.py

@@ -71,6 +71,11 @@
     balance_panel,
     create_event_time,
     generate_did_data,
+    generate_ddd_data,
+    generate_event_study_data,
+    generate_factor_data,
+    generate_panel_data,
+    generate_staggered_data,
     make_post_indicator,
     make_treatment_indicator,
     rank_control_units,
@@ -190,6 +195,11 @@
     "validate_did_data",
     "summarize_did_data",
     "generate_did_data",
+    "generate_staggered_data",
+    "generate_factor_data",
+    "generate_ddd_data",
+    "generate_panel_data",
+    "generate_event_study_data",
     "create_event_time",
     "aggregate_to_cohorts",
     "rank_control_units",