This branch adds the ability for the user to promote arbitrary slices (which can be made using NewScalarMask and NewVectorMask) into a custom Quantity. The implementation is backported from @XuebingZhao 's umagnus fork. Currently, custom fields can only be made from existing Quantities, Const or Constvector , limiting the amount of spatially varying Quantities that are possible.

• The main use case is that it allows users to create spatially varying field terms that are not limited to 256 regions, but of course there are many other options. Does not use any Cuda, so probably slower than Jakelove's Feature/spatial_quantity #289 implementation for spatial Quantities, but also probably more flexible overall.
• Works with both Scalar and Vector slices. Adding functionality to handle tensors isn't included and would be a bit of extra work, but users can fairly easily build a rank2 tensor out of Vectors.
• Works with normal Quantity operators (Add, Mul, etc), as well as with Run/Relax/Minimize etc.
• Included is a test file custom_quantities.mx3. I compared it to mumax's built-in uniaxial anisotropy, it matches exactly within numerical precision.
• In a previous PR, I mentioned potentially doing custom Excitations. After spending some more time on it, I believe it should actually be possible to do time varying Quantities as well, but it will take some more time to fully work it out and verify, so that will come at some later time in a different PR.