ENH: noiseModels - re-worked output directories to avoid clashes. Fixes

Both point- and surfaceNoise utilities can operate on multiple input
files.  However, if the files had the same name, the output would be
overwritten.  To avoid this, the output files are now written to a
sub-directory including the path '/input<input-file-index>/', e.g.

  postProcessing/noise/surfaceNoise/input0/nearWall/[fft|oneThirdOctave]

An optional 'outputPrefix' can be included (defaults to empty), e.g.
when set to 'test1':

  postProcessing/noise/test1/surfaceNoise/input0/nearWall/[fft|oneThirdOctave]