Model a frozen turbulent atmosphere

Calculate a snapshot of a frozen turbulent atmosphere.

Spectral densities have been implemented for Gaussian, Kolmogorov and
Von Karman spectra. Fluctuating fields can be calculated in 2D using
these three spectra. Wind and temperature fluctuations are supported.

1D and 3D fields are not yet implemented.

Only the classes that are used in the examples have been validated.
Merge pull request #9 from FRidh/turbulence

Model a frozen turbulent atmosphere