zero out execute_in to reveal a bug.
add functions to clear an array.
clean the input in all tests

This shows all of the r2c cases are broken by the conjugate code.
fix issue #4 in upstream.

when conjugate_in and conjugate_out are NULL, no conjugation
shall be performed.
Merge branch 'master' of https://github.com/rainwoodman/pfft into rainwoodman-master
do not clear input for inplace transforms
Merge branch 'rainwoodman-master'
do not clear input in stand alone forward FFT test for user manual