Support multiple ideal vertices in NS_QUAD_CLOSED coordinates.

[NathanDunfield]

Currently, the NS_QUAD_CLOSED coordinates are only available for ideal triangulations with a single ideal vertex. Nothing changes mathematically for multiple ideal vertices, and Regina already has the most of the code needed to handle that case. I was able get Regina to:

1. Support multiple ideal vertices with the older NS_VERTEX_DD algorithm by changing just a few lines of code.

2. Support up to 8 ideal vertices in the current best algorithm by this hack.

Both of these were tested extensively in the computations for this SoCG paper.

The difficulty with doing the second item cleanly is that LPConstraintNonSpun::nConstraints
is fixed at 2 at compile time, and that will be 2 * (num vertices) in the general case. In my hack, I just set nConstraints to 16.

Alternatively, one could have a separate LPConstraintNonSpun for each cusp, but as LPConstraintNonSpun is not actually instantiated it can’t store which cusp it is associated to.