enhancements to matset to_silo

[cyrush]

No description provided.

[cyrush]

@xjrc let me know what you think, FYI -- I want to improve some of the tests

[coveralls]

Coverage increased (+0.2%) to 86.5% when pulling 2559e1d on task/2021_01_matset_to_silo_updates into 51d0b71 on develop.

[cyrush]

One issue I need to resolve, the size of the matlist ends up being too big in the uni-buffer case:

topology: "topo"
material_map: 
  circle_a: 1
  circle_b: 2
  circle_c: 3
  background: 0
volume_fractions: [1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 0.333333333333333, 0.333333333333333, 0.333333333333333, 1.0, 1.0, 1.0, 0.5, 0.5, 1.0]
material_ids: [0, 0, 0, 0, 0, 0, 3, 0, 0, 1, 1, 2, 3, 2, 0, 1, 1, 2, 2]
sizes: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 1, 1, 1, 2, 1]
offsets: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 15, 16, 18]


topology: "topo"
material_map: 
  circle_a: 1
  circle_b: 2
  circle_c: 3
  background: 0
matlist: [1, 1, 1, 1, 1, 1, 4, 1, 1, 2, -1, 3, 1, 2, -4, 3, 0, 0, 0]
mix_next: [2, 3, 0, 5, 0]
mix_mat: [1, 2, 3, 1, 2]
mix_vf: [0.333333333333333, 0.333333333333333, 0.333333333333333, 0.5, 0.5]

(see the extra three 0s)

I think this value is coming from:

        blueprint::o2mrelation::O2MIterator mat_iter(mat_vfs);
        matset_num_elems = mat_iter.elements(O2MIndex::ONE);

I could just use the number of elements of the offsets or sizes, but I want to make sure the O2M stuff is working as we expect.

[xjrc]

Looks good Cyrus, but I think there's still one thing incorrect with my initial implementation; please take a look at my comment and let me know what you think. Once we get that issue resolved, I think we're ready to merge this.