only expose an opaque pointer for the different plans in the public API

[jenskeiner]

Transferred from: https://github.com/NFFT/nfft_old/issues/5

Right now, the public API exposes the whole plan structure, which increases the potential for API breakage in future updates. This is what happened when the _flags attribute has been renamed to just flags in a previous commit for instance.

In order to avoid that later, but also simplify the public API a bit, it would be better to only expose all plans as opaque pointers and hide the internal structure of each plan to the user, as it is done in FFTW actually.

The flipside of this change is that the x, f and f_hat can no longer be accessed directly and should be updated through the public APIs functions and / or assigned during initialization. I feel confident I can come up with a design for that following what is done in FFTW, but I wanted to discuss that here first.