Missing in-place real `rfft!` interface

[roflmaostc]

Hi,

since there is a PR for rfft! in FFTW we probably should also add rfft! in AbstractFFTs?

In general in-place real-to-complex FFT seems to be supported by CuFFT as well, right?

I recall that I was discussing this somewhere with @maleadt but I can't remember exactly.

What do you think about that?

Best,

Felix

[Luapulu]

Just want to add my two cents to say I'd like an in-place real to complex FFT for my application, too.