ENH Allow real values?

[eickenberg]

We could consider allowing real tensors for the finufft values that then are cast, possibly with the issue of a warning.

Similarly, it may be useful to allow to just specify values=1, or any float, which would then be broadcast to the correct dimension. This would be particularly useful if one is just interested in point locations. Could even consider making 1 the default argument, but that might lead to some unexpected behavior

[WardBrian]

Since this requires allocating something 2*sizeof(values) I think it's probably something we want to make the user explicitly do.

Hopefully finufft upstream gets a real specialization one day, and then we could accept reals and use that.