update the FCU-net model file directory structure

[alex-rakowski]

FCU-Net currently creates and downloads to tmp dir in the current working directory. We should update this to some global location so that multiple dirs can access to avoid needing to download weights in each dir.

This could be incorporated or should considered how if we handle a config file for global settings settings down the line, i.e. ~/.py4dstem/config.yml ~/.py4dstem/fcunet-weights/