Provide a Compact class to hold particle fuel and matrix

[drewj-usnctech]

Something that came up in a chat today: having a Compact composite class with strictly two children: a matrix Component and something to represent the particle fuel - #1055. The matrix would define the shape of the compact (cylinder, potentially spherical for pebble bed reactors). The Compact would / could be a child of a Block along with it's sibling Component children. Would require what we're calling arbitrarily deep composite tree (#1008) or at least a little deeper.

Thinking of the MHTGR-350 reactor (#224) you could think of a fuel element as a Block with the following children

block
  - circular coolant component
  - fuel compact composite
    - circular matrix component
    - particle fuel composite
      - spherical fuel kernel
      - spherical buffer
      - spherical ipyc
      - ...

Then, if the compact is placed on a lattice grid, the multiplicity of the compact is easy to get: number of lattice sites. And the matrix could have a multiplicity of (1 - packing fraction) to account for the volume occupied by the particle fuel. And the particle fuel (and subsequent layers) would have a multiplicity on the order of thousands to reflect the number of particles in one compact

Originally posted by @drewj-usnctech in #960 (comment)

[drewj-usnctech]

@john-science this and #1055 are something we at Tech are interested in exploring

cc @slee-USNC @cbroman-usnctech