Fix implementation of array-transposition.

FFTW code contained within #ifdef statements to allow compilation without requiring FFTW.

Only the innermost dimensions may be transposed using FFTW.  Higher-dimensions only support use of a copy array.
Update documentation for proper compilation of array classes.

No functional change.

Updated the *.pri file to add the source directory to the include path.  This means that the *.pri file only needs to be included by a *.pro file in order to use the code within that project.