use cupy.cuda functions instead of avail_gpu in pytom/gui/guiFunctions.py

[sroet]

          Looking at this whole function it would be better to switch to something like this : https://docs.cupy.dev/en/stable/reference/generated/cupy.cuda.runtime.getDeviceCount.html

This function even only allows 'GeForce' cards looking at line 482 :|

Originally posted by @McHaillet in SBC-Utrecht/PyTomPrivate#168 (comment)