implemented low memory option for fid.py

[Pyrestone]

See #14

I implemented behaviour to load and un-load each batch of images during evaluation of the activations instead of loading all images to RAM at the beginning of evaluation.
This reduces memory requirements by a significant amount.
Specifically, it reduced the memory footprint from >16GB to <4GB when using 10.000 images of size 256x256.

The new behaviour is triggered by enabling the --lowprofile flag in the command line args of fid.py

I made this the non-default behaviour since it might increase execution time slightly in non-memory-constrained environments.

The results should be near-identical.
I have observed small differences between scores, at non-significant digits (5th place after comma), but those are probably explained by floating-point rounding errors.

[untom]

LGTM, thanks for the contribution 👍 :)