Enable the ability to save the RRDN model

[thekevinscott]

Pursuant to this comment thread, enables the ability to save the RRDN model.

I tried to implement a get_config but was unsuccessful, so I copied the implementation by @gkrislara. I'm using this code to save an RRDN model successfully.

Sample code for saving:

from ISR.models import RRDN
model = RRDN(weights='gans')
model.model.save('./image-super-resolution-gans.h5')

[thekevinscott]

I've updated the PR to use custom layers; these also leverage get_config as referenced above.

[uiyunkim-git]

Just adopted your rrdn.py

The error "can't pickle _thread.RLock objects" has fixed.

Thanks!

[cfrancesco]

looks great, thanks!