Beam `direction` is confusing

[dagewa]

Inspired by #868 I offer another piece of perennial confusion in the dxtbx model. We create a Beam model like this:

dxtbx/tests/model/test_beam.py

Lines 19 to 20 in 55fa1ba

	# Create the beam
	b = Beam(direction, wavelength)

I suspect any sensible developer new to the project would assume that direction means the direction that the photons/electrons/neutrons travel. No, no! It means the sample-to-source direction, which I find totally perverse (I suspect the history of this is to do with some kind of over-zealous alignment to imgCIF conventions)