Geometry module and orix interface

[jadball]

Added ImageD11.sinograms.geometry

Should partially address #269 although some tasks are still outstanding

Added orix interface to unitcell class

• unitcell.orix_phase is now a property of unitcell if it was created with an integer spacegroup symmetry.
• added unitcell.get_orix_orien() - computes the orix orientation from a provided numpy array of UB matrices. Note that this can be more than one UB matrix! - the orix orientation class can contain multiple orientations.
• added unitcell.get_ipf_colour_from_orix_orien() - computes the IPF colour from an orix orientation given a corresponding axis
• added unitcell.get_ipf_colour() - computes the IPF colour from a numpy array of UB matrices
• grain objects now have a ref_unitcell property to supply a unitcell class instance
• grain objects have helper methods for getting orix_phase and orix_orien from grain.ref_unitcell
• added a few unit tests for the orix interface in test/test_orix.py - should not break CI as will be skipped on Py2
• added/updated IPF and PF plots in nb_utils.py to use new orix interface - much faster and more reliable now!
• added demonstration notebook for IPF and PF plots in sandbox/orix/test_pole_figures.ipynb
• added orix and diffpy.structure as 'useful' dependencies in setup.py for python_version >= "3"

It's not all sunshine and roses though - unfortunately I've been unable to get a unit test to work with a large strain between B (e.g the B matrix of a measured grain) and B0 (the B matrix used by the reference unit cell). I think the problem is that with large strains, there is a significant angle between [a* of the B0 matrix in the lab frame] and [a* of the B matrix in the lab frame]. I will make a separate issue to track this once merged.

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