Changes to support compound materials layers

[adrianaghiozzi]

See related PRs in FusionMaterials, IMAS, and IMASDD. These changes allow build layers to be made of multiple materials, where each element in the material vector has a name and associated composition.

There are still a few things to be done:

• Update build.tf.technology.material and build.oh.technology.material to also allow for compound materials and map those to the correct build layers in init_build
• I've added a field called "fabrication" under dd.build.layer[:] that could eventually hold a symbol that describes the fabrication of the compound material itself - i.e. is it :homogenized, :layered, :corrugated, etc. The field is there but at the moment it's not hooked up to anything.
• For now, I removed the setproperty! that disallows certain materials from being assigned to certain layers. We could eventually rethink this to be compatible with the compound layers
• There is now a function in FusionMaterials called compound_material_property that calculates compound material properties as a simple linear combination of the materials it's comprised of. @bclyons12 will need an extension of this that works for materials connected in parallel rather than in series for vertical stability, and we could add that function to FusionMaterials for others to access

[adrianaghiozzi]

@bclyons12 Note that I put a sketchy fix here in the vertical stability actor just to avoid getting errors while I was testing this - as it is this would only take the first material in the list and set that as the vacuum vessel material. Eventually when you actually have compound layers you'll want to fix this.