[ENH] Set dual amp thresholds with other thresholds #142

Currently, amp_threshes=(1, 2) kwarg must be passed to the burst_kwargs argument in order for it to be passed to ndsp's detect_bursts_dual_threshold. Currently, this is how amp_threshes must be set:

amp_thresholds = (1, 2)

burst_thresholds = {
   'burst_fraction': 0.5
}

bm = Bycycle(
   burst_method='amp',
   thresholds=burst_thresholds,
   burst_kwargs={'amp_threshes': amp_thresholds}
)

Instead of passing the amp_thresholds with burst_kwargs, it would make sense allow these to be set from the thresholds kwarg. Something like below, should be equivalent to above:

burst_thresholds = {
    'burst_fraction': 0.5,
    'amp_enter': 1.,
    'amp_exit': 2.
}

bm = Bycycle(
    burst_method='amp',
    thresholds=burst_thresholds
)