Fix `ift.Field.vdot` breaking with np.uint32 and other integer types

Hi Martin,

I just stumbled over ducc0.misc.vdot not multiplying doubles and integers, which is required in the PoissonianEnergy. It seems Philipp Arras did so, too, a while ago, but unfortunately his mitigation was incomplete, as it did only catch np.int64s.

Is there a double*integer vdot in ducc0 that we could call instead of the conversion?

While at it: I noticed ift.PoissonianEnergy enforces integer-typed data fields (sensible for Poisson counts), but then passes them directly to vdot, triggering an int->double conversion in each application. Should we cast the data field to double in the initialization of PoissonianEnergy after the integer check to avoid this?

Cheers, Lukas