Vortex stretching is one of the dominant terms in both the
vorticity equation and enstrophy equation. They also help in
understanding the vortex dynamics and how energy at large scales
cascades into small scales.

The three components of vortex stretching (omega dot grad) u
and the enstrophy stretching production (omega_i omega_j S_ij)
are now included and can be computed with the derivatives
case file. Files for vortex stretching terms are called
vort-stretch<dim> where <dim> is one of x,y,z. The enstrophy
stretching production file is called enst-stretch.

As these quantities are the product of velocity gradients, care
must be made to ensure adequate resolution is present.

Update version to 2.0.3