Combine quality calculations into single function

[slhck]

Create a new function that combines all quality metrics into one function to only have to scale/read everything once.

The user can then supply the required options … possible syntax:

def calc(metrics=["ssim", "psnr"], vmaf_options={})

Where metrics can be any supported metric, and vmaf_options is a dict with the metric-specific options (no such options for other metrics).