Support for NTuple inputs and outputs

[heltonmc]

It would be nice if the example function could also work for NTuple types such as

@. model(x, p) = p[1]*exp(-x*p[2])
xdata = (0.5, 1.5, 2.5, 4.5, 6.5)
ydata = model(xdata, (1.0, 2.0))
p0 = (1.5, 0.5)
julia> fit = curve_fit(model, xdata, ydata, p0)
ERROR: MethodError: no method matching curve_fit(::typeof(model), ::NTuple{5, Float64}, ::NTuple{5, Float64}, ::Tuple{Float64, Float64})

I've found that I'm starting to use Tuples significantly more in math models for speed/allocations when considering small number of parameters.

My current workaround has been to do...

@. f(x::Tuple, p) = p[1]*exp(-x*p[2])
xdata = (0.5, 1.5, 2.5, 4.5, 6.5)
ydata = f(xdata, (1.0, 2.0))

model(x, p) = [f(Tuple(x for x in x), p)...]
p0 = [1.5, 0.5]
fit = curve_fit(model, [xdata...], [ydata...], p0)

Which works but obviously allocates a lot more. Is there perhaps a better workaround for this if f must accept a tuple? Possibly just use the inplace version?