Add a separate default detector & fix threshold range for ema detector

1. currently when bitmap detector throws an exception due to lack of data points, exponential moving average(ema) detector acts as the default detector. Adds a default detector which uses both ema detector and derivative detector.
2. The 10% default threshold is too low for ema detector. Change score method and use a different threshold.

you mean in the code? the correlation between the following two series will be high if the anomaly score are not denoised:
[0.08, 4.6, 4.6, 4.6, 1.0, 1.0]
[0.0010, 0.0012, 0.0012, 0.0008, 0.0008] in which case the second series is a flat line and the ano