clean up and save fft panel

[weiyuan-jiang]

Summary of changes:

1. FFT for generation of perturbations:
   a) check the status of fft calls
   b) save fft panels to avoid initializiing and finalizing fft environment every time
2. added cat_diagS_max function to avoid NaN issue when computing ens std-dev Possible NaN is introduced #678
3. clean up of unused variables

All LDAS nightly tests passed (incl. Intel, aggressive, and GNU) after most recent commit b5fe7b1