super_resolution: add freeze_safe support, python lint issues, checkpoint_every_n option

[jefflomax]

This is a simple update to super_resolution so that:

It implements standard "freeze_support" so multithreading works on both Windows and Linux Removes Python-Lint warnings by passing references to variables Adds the option to store a checkpoint every N epochs, defaulted to 1 Without these changes, the sample does not run on Windows.