chore: dead code cleanup - remove ~2,450 lines of commented code (#419)

* feat(tests): add debug_print fixture for toggleable test output

Add pytest --debug-prints flag and debug_print fixture for controllable
debug output in tests. This enables preserving valuable debug prints
while keeping normal test output clean.

Usage:
  pytest tests/                    # Silent
  pytest tests/ --debug-prints -s  # With debug output

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

* chore: remove unused import pdb statements from 35 files

Phase 2 of dead code cleanup. These pdb imports were added for debugging
but never used (all had `# noqa: F401` to silence flake8 warnings).

Files modified across: plotting/, fitfunctions/, core/, solar_activity/,
instabilities/, tools/, and root __init__.py

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

* chore(plotting): remove 1,830 lines of dead code from histograms.py

Phase 3 of dead code cleanup. This file contained legacy implementations
of AggPlot, Hist1D, Hist2D, and GridHist2D classes that were fully
commented out. The actual implementations now live in their own modules
(agg_plot.py, hist1d.py, hist2d.py).

File reduced from 1,845 lines to 15 lines (99% reduction).

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

* chore: remove 584 lines of dead code from 5 source files

Phase 4 of dead code cleanup. Removed commented-out implementations,
debug prints, and memory profiling code.

Files cleaned:
- plasma.py: -235 lines (commented algorithms, debug prints)
- base.py: -153 lines (old clip_data, Plot2D class)
- spiral.py: -90 lines (timing/debug code)
- hist2d.py: -92 lines (old implementations)
- orbits.py: -14 lines (memory profiling)

Preserved all TODO comments as active work items.

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

* chore: remove additional dead code from orbits.py and test prints

Phase 5 of dead code cleanup:
- orbits.py: Remove 59 lines of commented logging/debug code
- test_performance.py: Remove 15 lines of timing benchmark prints

Total additional cleanup: 74 lines removed

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

* style: fix black formatting in __init__.py

Removed extra blank line after docstring that black flagged.

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

* style: fix black formatting in data/reference/__init__.py

Fixed blank line formatting from merge with master.

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

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: blalterman

solarwindpy/__init__.py

@@ -4,8 +4,6 @@
 context (e.g. solar activity indicies) and some simple plotting methods.
 """
 
-import pdb  # noqa: F401
-
 from importlib.metadata import PackageNotFoundError, version
 
 import pandas as pd