installation: new local FFT file cache

* Adds `populate-fft-file-cache.sh` helper build script useful to
  pre-download all files used in FFTs.  (closes #350)

* Amends installation instructions on how to use local FFT file cache.

* Amends all XML input files accordingly.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>