Create default visualization parameters for each sensor

[earth-chris]

In mapping Earth Engine image data, you pass the Map.addLayer function a dictionary with visualization parameters.

We should provide an easy way to get a standard dictionary with sensor-specific visualization parameters, probably similar to how the getBands() function is implemented in ccblc/utils.py.

My preference would be to visualize [swir1, nir, red] as RGB values as the primary visualization scheme, then [nir, red, green] as the backup. One example of default visualization parameter dictionary for Landsat 8 would be:

visParams = {
  'min': 0,
  'max': 0.45,
  'bands': ["B6", "B5", "B4"]
}