FFTPower shall support unique binning.

[rainwoodman]

FFTPower shall support unique binning where each bin is guarenteed to have at most one |k|.

This is to ease comparison with theory curves, because

<P(k)> != P(<k>)

[rainwoodman]

PR #497 helps this situation without getting into details of implementing the unique binning.

The current issue is that kedges are computed before the 3d mesh is generated (inside compute_3d_power); but the k modes are only available during that function. (When ComplexField is created). It should be simple to hack this.

I checked that for 3D modes, the number of unique |k| values scales roughly like N^2.15. So for a 1024 mesh it should be less than a million unique |k| modes. This is definitely a resonable size.

[rainwoodman]

This is already supported.