Generalise NURBS weights

[rafaqz]

Currently NURBS weights are a single contiguous array. But with dimension generalisation coming in #33 this no longer really fits the package structure. E.g. what if the first and last dimensions have weights but not the second and third.

One solution would be to add or require input weights to have length one dimensions for all dimensions without weights. Another is to keep the array the same just filter the dimensions whenever using weights (that's a compile-time op anyway).