Usage:
  alice3 ...
  where ... indicates any combination of the optional parameters below.

  examples:
    alice3 in=wmap_ilc_3yr_v2_r6_sim.fits nside=512 ell=300 out=test

  NOTE: the syntax is slightly different from the one used in alice2!
  There must not be any spaces on either side of the "=" signs!

  [in=<fits file>] FITS file containing the Q and U polarization
  components in the second and third columns.

  [out=<string>] string used as a prefix for output files
  alice3 generates three different output FITS files:
  <out>_background.fits: the texture before the LI convolution
  <out>_texture.fits: the texture after LI convolution
  <out>_mod_texture.fits: the convolved texture, modulated with
    polarization intensity
  By default the code will exit with an error instead of overwriting
  an existing FITS file! If you want to override this behaviour,
  prepend your prefix with an exclamation mark (which may need to be
  protected from shell expansion by a leading backslash).

  [texture=<fits file>] optional input background texture
  if left empty, a texture is generated using the "ell" parameter

  [nside=<int>] nside of all generated maps

  [ell=<int>] ell value at which the generated background texture has power
  if ell==-1, use ell=2*nside
  if not supplied at all, use a Healpix map with given Nside containing white
    noise

  [rand_seed=<int>] seed for the random number generator
  (default=42). Only used when a texture is generated internally.

  [steps=<int>] number of steps to use for each line of the convolution
  (default: 100)

  [kernel_steps=<int>] extent of the convolution kernel (in steps)
  (default: 50)

  [step_arcmin=<float>] arcminutes for each step in the convolution
  (default: 10)

  [polmin=<float>] minimum value for the polarization magnitude

  [polmax=<float>] maximum value for the polarization magnitude
