Add a diffraction module for handling of reciprocal lattice points and structure factors

[hakonanes]

Description

A diffraction module to enable easy creation, storage and manipulation of reciprocal lattice points (Miller indices/Kikuchi bands), which stores

• Miller indices
• direct space interplanar spacing (d-spacing)
• reciprocal space lattice point spacing (g-spacing)
• structure factors
• similar quantitites

The class is loosely based upon EMsoft's gnode type (https://github.com/EMsoft-org/EMsoft/blob/develop/Source/EMsoftLib/typedefs.f90#L1238).

It needs a well defined crystal structure as a orix.crystal_map.Phase object, which conveniently stores diffpy.structure.Structure, diffpy.structure.SpaceGroup and orix.quaternion.symmetry.Symmetry objects.

The reciprocal lattice point indices are stored as orix.vector3d.Vector3d objects, which allows use of the Vector3d.unique() method to get unique indices, compute the multiplicity and other useful things.

Checklist

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean style in as per black
• Loads more, will add with time

Minimal example of the bug fix or new feature

>>> from orix.crystal_map import Phase
>>> from diffpy.structure import Lattice, Atom, Structure
>>> from kikuchipy.diffraction import ReciprocalLatticePoint
>>> lattice = Lattice(2.8665, 2.8665, 2.8665, 90, 90, 90)
>>> atoms = [Atom("Fe", [0, 0, 0]), Atom("Fe", [0.5, 0.5, 0.5])]
>>> structure = Structure(lattice=lattice, atoms=atoms)
>>> p = Phase("ferrite", space_group=229, structure=structure)
>>> rlp = ReciprocalLatticePoint.from_min_dspacing(p, 0.5)  # Ångstrøm
>>> rlp
ReciprocalLatticePoint (19,)
Phase: ferrite (m-3m)
[[3 3 3]
 [3 3 2]
 [3 3 1]
 [3 3 0]
 [3 2 2]
 [3 2 1]
 [3 2 0]
 [3 1 1]
 [3 1 0]
 [3 0 0]
 [2 2 2]
 [2 2 1]
 [2 2 0]
 [2 1 1]
 [2 1 0]
 [2 0 0]
 [1 1 1]
 [1 1 0]
 [1 0 0]]
>>> rlp.multiplicity
array([ 8, 24, 24, 12, 24, 48, 24, 24, 24,  6,  8, 24, 12, 24, 24,  6,  8,
       12,  6])
>>> rlp.dspacing
array([0.55165818, 0.61113985, 0.6576202 , 0.67564053, 0.69522837,
       0.76610435, 0.79502406, 0.86428227, 0.90646689, 0.9555    ,
       0.82748727, 0.9555    , 1.01346079, 1.17024372, 1.28193777,
       1.43325   , 1.65497455, 2.02692159, 2.8665    ])
>>> rlp.gspacing
array([1.8127167 , 1.63628668, 1.52063455, 1.48007699, 1.43837629,
       1.30530521, 1.25782357, 1.15702941, 1.10318425, 1.04657248,
       1.2084778 , 1.04657248, 0.98671799, 0.85452285, 0.78006907,
       0.69771498, 0.6042389 , 0.493359  , 0.34885749])
>>> rlp.calculate_structure_factor()  # Don't think this is correct yet
>>> rlp.structure_factor
array([1.8127167 , 1.63628668, 1.52063455, 1.48007699, 1.43837629,
       1.30530521, 1.25782357, 1.15702941, 1.10318425, 1.04657248,
       1.2084778 , 1.04657248, 0.98671799, 0.85452285, 0.78006907,
       0.69771498, 0.6042389 , 0.493359  , 0.34885749])
>>> rlp[rlp.structure_factor > 0]
ReciprocalLatticePoint (9,)
Phase: ferrite (m-3m)
[[3 3 2]
 [3 3 0]
 [3 2 1]
 [3 1 0]
 [2 2 2]
 [2 2 0]
 [2 1 1]
 [2 0 0]
 [1 1 0]]

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  the unreleased section in doc/changelog.rst.

[hakonanes]

This functionality will be moved to diffsims (see discussions in pyxem/diffsims#80). kikuchipy will then depend on diffsims for this functionality.