Removed (mostly unused) ExSeisFlow to improve maintainence burden of the project.

Reimplemented or removed a number of apps and examples that used ExSeisFlow
for their implementation.

Removed code using FFTW3 to eliminate the dependency.

Change Log
* Remove ExSeisFlow
* Split piol.hh into utils.hh and piol.hh
* Remove a number of unused files
* Reimplement apps/sort.cc and apps/concatenate.cc using PIOL
* Remove ExSeisFlow examples
* Remove code depending on FFTW3