Replace pos_array and freq_array with into and fft_array constructors

[SJSeckmeyer]

Having an explicit function, still written in the way of "Give me the array in the following space" would make it easier to allow transforms of just some dimensions and not all of them, which would be interesting for example for combined time and space arrays. Additionally it would make the integration with named array libraries like xarray(see #38) more natural in the future.

[SJSeckmeyer]

Discussion result, approved with API:

x=FFTDimension("x", ...)
V=0.5*m*w**2*x.fft_array(space="pos")**2
V_GPE = np.abs(psi.into(space="pos"))

[SJSeckmeyer]

Closed with #60.