Underlying Triangle Data Structure

[alecloudenback]

Some thoughts:

• may be better to have triangles be arrays of arrays without missing data.
  • It's inefficient from a data layout perspective to have data run horizontally if operations are primarily on horizontal rows because Julia is column-oriented
  • No reason to store data that doesn't existi as missing (see, e.g. SparseArrays which only store the nonzero values but still act like matrices.
• 4D structure
  • Is this just an array of triangles? give each triangle a(n optional?) index name?