diffraction.line: Diffraction Line calculation
In tjconstant/photonMonkey: Misc Tools for Photonics

Return a diffraction line to be added to a plot, given a vector of in-plane wavenumbers (k)

1	diffraction.line(k, kgx, kgv, m, n, phi = 0, alpha = pi/2)

`k`	in-plane wavenumbers.
`kgx`	the grating vector of the (phi=0) periodicity (2*pi/pitch)
`kgv`	the grating vector of the out-of-plane periodicity (2*pi/pitch)
`m`	in-plane scattering integer
`n`	out-of-plane scattering integer
`phi`	azimuthal angle away from kgx (phi=0 along kgx) in radians
`alpha`	angle between the two grating vectors in radians

##---- Should be DIRECTLY executable !! ----
##-- ==>  Define data, use random,
##--	or do  help(data=index)  for the standard data sets.

## The function is currently defined as
function (k, kgx, kgv, m, n, phi = 0, alpha = pi/2) 
{
    return(data.frame(k = k, omega = 3e+08 * (sqrt(((k * cos(phi) + 
        m * kgx - n * kgv * cos(alpha))^2) + ((k * sin(phi) - 
        n * kgv * sin(alpha))^2)))))
  }