Support callable objects in curve_fit

[jl-wynen]

Currently, this

class Model:
    def __call__(self, x, *, param):
        return x * param

curve_fit(Model(), x, p0={'param': sc.scalar(1)})

raises

ValueError: Fit function must take exactly one positional argument

But this should be allowed.

This comes from the requirement that fit parameters must be passed as keyword arguments. But it limits the use to functions.

Note that curve_fit(Model().__call__, x, p0={'param': sc.scalar(1)}) has the same problem. We need to define a wrapper function or lambda to workaround the limitation.