CCDCesque: masking improvements

[ceholden]

Make the multitemporal masking more configurable by doing the following:

• Parameterize the span of multitemporal cloud filtering when using smooth_mask / lowess filtering.
• Allow the critical values to be specific to the green & swir bands
  • User can pass a float or a tuple; float transformed to tuple of (float, float)
• Allow the critical values to be data-driven
  • Example, 2 * np.std(y) where y is the green or swir1 bands
  • Blocker: I have no idea how to be clever about passing and evaluating a str containing a mathematical formula (Patsy won't cover all of the maths we might want to do)