amplitude threshold burst detection with cycle discretization does not work for trough-centered cycles

[Scott Cole]

burst.detect_bursts_df_amp could be easily modified to work for peak-to-peak cycles.

'sample_last_trough' is hard coded, but the code could just determine which extrema are the centers and sides.

[Ryan Hammonds]

This was completed in #63 in the compute_burst_fraction function of bycycle.features.burst, the diff is here.