PCA-CD: modify sample_period based on density estimate

[macandleighs]

Original TODO

modify sample period dependent upon density estimate
    line 97 (initialization of self.sample_period) in pca_cd.py

Other Description

sample_period (float, optional): how often to check for drift. This is 100 samples or sample_period * window_size, whichever is smaller. Default .05, or 5% of the window size.

Change initialization of value for sample_period in PCA_CD based on density estimate. Currently defaults to 5% of the window size.