Accept or reject matrix input

[mcabbott]

As discussed here: https://discourse.julialang.org/t/error-in-mul-c-a-b-if-size-b-n-1/47999

At present curve_fit(model, xdata::AbstractArray, ydata::AbstractArray, p0::AbstractArray; ...) looks like it accepts arrays, and claims to accept arrays in its docstring, but in fact seems to only accept vectors.

It would be less confusing if the signature was ::AbstractVector, but even better, it looks like adding a few vec calls might easily allow ::AbstractArray to just work.