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.

I mean in the method docs (here). E.g., I didn't know why we needed to denoise the data, so it will be helpful to document the reasoning here.