phantom+mcfost: option to return opacities, optical depths etc for SPH particles

[danieljprice]

would be nice to have the option to return MCFOST-computed quantities back to phantom, such as Rosseland or Planck-mean opacities, optical depths, UV and X-ray fluxes for each particle

this would need some work to pass these through the library interface, but could then be written into phantom dump files to aid analysis