VisPy plotting of unit cells and magnetic structure in 3D

[RichardWaiteSTFC]

Use VisPy library to plot unit cells and magnetic structure - this will ultimately replace MATLAB graphics which can't be used in PySpinW on mac

Requirements

• [x] Plot atom positions in a unit cell (from sw.atom)
• [x] Support non-orthogonal cells (see sw.basisvector)
• [x] Plot crystal axes object
• [x] Plot polyhedra (see plotchem)
• [x] Plot exchange couplings (from sw.coupling)
• [x] Plot DM bonds (see sw_mattype)
• [x] Plot single-ion anisotropy/g-factor ellipsoid (see plotion)
• [x] Plot moment directions
• [x] Plot rotation plane of moments
• [x] Plot multiple unit cells/supercells
• [x] Plot fractions of cells (e.g. https://spinw.org/tutorials/09tutorial)
• [x] Add a legend/labels

Implementation

• [x] Add VisPy to dependencies
• [x] Add interface to spinw objects

[RichardWaiteSTFC]

Progress so far

[RichardWaiteSTFC]

Plotting half a unit-cell along c-axis of crystal with non-orthogonal basis vectors

[RichardWaiteSTFC]