NPA

## Improve MC NPA routine
* Realistic NPA detector defined by radii of aperture and detector and separation between them
* Can handle more than one npa chord at a time
* Only does calculation in cells that neutral particles can reach, improves run time tremendously e.g. 3 chords in 1.5 hours with 100000 particles running on 4 cores
* Outputs flux as a function of energy

## NPA weight function 
* very fast 2-3 min for 3 chords on 4 cores
* outputs flux and velocity weight function
* agrees with MC NPA

## Other changes
* added a switch to load fbm b/c NPA weight function can be run with or without it