Removing dependence on layout of dual simplices

[lukem12345]

          Checking that this is the dual vertex that is at the center of a primal vertex may just "fall out" of using our accessors in a certain order. So the following reliance on checking if it is in 1:nv(sd) may be unnecessary.

Originally posted by @lukem12345 in #95 (comment)

[lukem12345]

Currently, certain operators in the FastDEC module are made speedier by relying on dual simplices being added into the dual complex in a certain order at construction time. This can sometimes lead to big speedups.

However, relying on the construction order is inherently a more fragile approach than using our more generic accessors.

So, we should re-review the FastDEC module and see where this fragility can be eliminated.