Add functions for accurate FRET corrections
Add fretmath to __init__.py
Expand fretmath and add new tests
Compatibility: import gamma_correct_E() in burstlib from fretmath
Refactor: use leakage instead of BT or bleed_through
Docs: update notebooks
One more BT -> leakage conversion
Add FRET fit through KDE local maximum position
Add multi-ch E fit using KDE max position
Change default KDE bandwidth to 0.025 in fret fit.
Add --nogui flag to load_fretbursts to avoid freezes when using cells->run-all in ipython notebook.
Add bg_from() as standard method to obtain bg rates for any ph_sel
Small refactor of the select_bursts.ES* functions.

- Rename: ES -> ES_rect, ESe -> ES_ellips
- New function ES for both rectangular or elliptical selection
- Docstring fixes
Add plot_ES_selection() to show selection regions on E-S plots
Add parameter to hist2d_alex() to control colorbar range.
Cleanups on ALEX histogram computation
Minor docstring cleanups in burst_plot.py
Update scatter_alex() plot style
Add 'nd+na' to hist_size
Fix exception when calling update_* methods before burst search