Allow the change of k in fixed_threshold

[sarahmish]

Orion/orion/primitives/timeseries_anomalies.py

Line 145 in 09b11ef

def _fixed_threshold(errors, k=4):

Currently, fixed threshold is always set on k=4 which makes the threshold set on four standard deviations away from the mean. We would like to make k dynamic and changeable.

Proposal:

• if fixed_threshold is False, go to dynamic thresholding
• if fixed_threshold is True, make the fixed threshold set to a default which is k=4
• if fixed_threshold is an int or float, make this an indication on how far away from the mean it should be