Fix XRayTransform2D projection dtype and docs

[Michael-T-McCann]

resolves #542
hypothesized to resolve a warning about dtypes from the tests

[bwohlberg]

I noticed that the public versions of these methods

scico/scico/linop/xray/_xray.py

Lines 126 to 132 in c58b819

	def project(self, im: ArrayLike) -> snp.Array:
	"""Compute X-ray projection."""
	return XRayTransform2D._project(im, self.x0, self.dx, self.y0, self.ny, self.angles)
	def back_project(self, y: ArrayLike) -> snp.Array:
	"""Compute X-ray back projection"""
	return XRayTransform2D._back_project(y, self.x0, self.dx, self.nx, self.y0, self.angles)

are not fully document, while the private versions are. It wouldn't hurt to add the missing docs in this PR.