Type promotion

It seems interpolating static arrays of eltype Float32 causes a type promotion to Float64:

```
julia> interpolate([@SVector(rand(Float32, 3)) for i=1:5], BSpline(Linear()))
5-element interpolate(::Array{SArray{Tuple{3},Float32,1,3},1}, BSpline(Linear())) with element type SArray{Tuple{3},Float64,1,3}:
Float32[0.40708745, 0.08114183, 0.64380157]
Float32[0.889678, 0.40728843, 0.3275386]
Float32[0.064697385, 0.27480924, 0.08783579]
Float32[0.21843159, 0.546651, 0.7467506]
Float32[0.6662456, 0.8931141, 0.17646503]
```

It says: `with element type SVector{3,Float64}`

Is that expected?

It seems to be hitting `tweight(A::AbstractArray) = Float64`.