Added r2c_one spec
Referenced correct part of complex array

As noted in http://www.fftw.org/doc/Complex-numbers.html#Complex-numbers, the
[0] element holds the real part and the [1] element holds the imaginary part.
Corrected argument number for size
Registered size of both input and output matrices.
Moved the output matrix write operation to after the plan execution
Corrected typo of double-semicolon
Added an output matrix argument to r2c_one