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