Scalar method requires element-wise evaluation of array-valued functions

I have a function definition that solves a system of equations by calling `gesv!`.
If the system is large, `gesv!` is called multiple times for each solution element.
This is because the current FFT call in '_wcoeff' only handles scalar valued functions.

I can make a seperate method that deals with array valued functions by storing the Laguerre-coefficients in higher dimensional arrays and applying a FFT along the first dimension of those arrays.

For simplicity, I'd start with vector valued functions.