Add vector output support for AkimaInterpolation

[jbatteh]

Is your feature request related to a problem? Please describe.
Currently AkimaInterpolation only supports output of a single value so it cannot output a vector of values. This support is available for LinearInterpolation, CubicSpline, etc.

A clear and concise description of what the problem is. Ex. I’m always frustrated when […]
AkimaInterpolation only supports a single vector for interpolatin unlike some of the other methods implemented.

Describe the solution you’d like

The following should work as it does for other methods:
u = vcat(2.0collect(1:10)', 3.0collect(1:10)')
t = 1.0collect(1:10)
A = AkimaInterpolation(u, t)

Additional context

It might be nice if the docs indicate which methods support vector output and which do not.