Fixed normal modes transformation

The full diff view is a bit too much to read, the individual commits are better. Here is the list of changes:

* Use the FFT wrapper, rather than platform-specific FFT libraries for the PIMD normal modes transformation.
* Clean up unused variables in several functions.
* Some readability and style changes in multiple places.
* Restructure if-else in control_pimd_trans.c, no functionality change. 
* Move debug code to control_pimd_trans.c