Re-type Topologic Helper Functions

[lukem12345]

          This docstring was confusing: you mean returns a vector where the element at index `i` is a set of simplices of dimension `i - 1`. This should probably be a tuple rather than a vector, and you should wrap it with a struct with a good name, like

struct Simplices{Ts <: Tuple}
  by_dimension::Ts
end

Originally posted by @olynch in #59 (comment)