Revise where `xp` should and shouldn't be used

[yousefmoazzam]

The xp variable was introduced to deal with places where either numpy or cupy would work.

However, over time, the usage of xp has crept into places where what is being accessed would not work if xp = numpy. I think it would be better to not overly rely on xp where possible, so combing through the repo and weeding out unnecessary/incorrect uses of it would be helpful.

Two examples

1. using xp.cuda.runtime.getDeviceCount() will not work if xp = numpy

httomo/httomo/method_wrappers/generic.py

Lines 128 to 131 in b28c2c3

	if gpu_enabled:
	self._num_gpus = xp.cuda.runtime.getDeviceCount()
	_id = httomo.globals.gpu_id
	self._gpu_id = mpiutil.local_rank % self._num_gpus if _id == -1 else _id

2. using xp.cuda.Event() will not work if xp = numpy

httomo/httomo/utils.py

Lines 271 to 276 in b28c2c3

	class catch_gputime:
	def __enter__(self):
	if gpu_enabled:
	self.start = xp.cuda.Event()
	self.start.record()
	return self