Implement Ph_sel according to issue #1
Fix semantic of Data attribute ph_sel, add attribute bg_ph_sel.

With this commit the attribute ph_sel stores the ph selection used for burst search.
A new attribute bg_ph_sel stores the selection used by Lim and Ph_p
Update calc_bg() docstring to use better ph_sel names
Fix names of Data attribute bg_th_us_*
Remove ' from the git commit line
Update remaining modules to use Ph_sel object
Fix bg_calc() using 'auto' threshold and non-ALEX
Add tests for bg_calc and burst_search_t using Ph_sel and 'auto' bg threshold
Fix semantic for  background period limits recomputation
Remove redundant background thresholds saved in Data()
Change print in recompute_bg_lim_ph_p()
Merge branch 'ph_sel'. Closes #1.
Update regression tests