Update trial factor

It has been agreed (suggested) that we should send a preliminary alert for any event with a reported FAR < 2/day for usage by partners.

We should set up the thresholds and trial factor accordingly

CBC searches = spiir-allsky/gstlal-allsky/mbta-allsky/pycbc-allsky + burst-cwb-bbh + mbta-ssm

BURST searches = cwb-allsky/olib-allsky + (not active) MLy-allsky

Trial factors and thresholds are set by:

significant_alert_far_threshold = {
    'cbc': 1 / (30 * 86400), 'burst': 1 / (365 * 86400), 'test': 1 / (30 * 86400)}
significant_alert_trials_factor = dict(cbc=5.0, burst=4.0)
preliminary_alert_far_threshold = {
    'cbc': 2 / (1 * 86400), 'burst': 2 / (1 * 86400), 'test': 2 / (1 * 86400)} 
preliminary_alert_trials_factor = dict(cbc=8.0, burst=8.0)
early_warning_alert_far_threshold = 1 / (30 * 86400)
early_warning_alert_trials_factor = 4.0

Related Issue: #639 (closed)