ENH: noiseModels - replaced graph usage by writeFile

Header information now includes, e.g.

f [Hz] vs P(f) [Pa]
Lower frequency: 2.500000e+01
Upper frequency: 5.000000e+03
Window model: Hanning
Window number: 2
Window samples: 512
Window overlap %: 5.000000e+01
dBRef       : 2.000000e-05
Area average: false
Area sum    : 6.475194e-04
Number of faces: 473

Note: output files now have .dat extension

Testing: check output from

$FOAM_TUTORIALS/incompressible/pimpleFoam/LES/vortexShed