Isosurface performance enhancements

[simonster]

This gives a 6x speed boost with a large volume on my system. The changes:

• Avoid calling collect on the whole Dict, which was very expensive because of all the heap-allocated tuples. Instead just iterate over keys to construct vtD and collect values to construct vtAry.
• Avoid allocating temporary arrays, replacing them with tuples where that simplifies the code.
• Remove expensive vId inner function. Unfortunately this results in some ugliness, but I think the performance gain is worth it.

[coveralls]

Coverage decreased (-0.15%) when pulling dd8142c5343ac0efdba79032f533963efd49ca37 on simonster:perf into 8b2e55545117225023f164ac11a2fc9f0c3a148a on twadleigh:master.

[twadleigh]

@simonster, thanks for showing some code love. Looks good to me. @sjkelly, good to pull?

[sjkelly]

Agreed, looks good. I haven't seen the NTuple type before. This is handy!

[simonster]

Thanks for reviewing!