[ganjinzero1]

Add a few new functionality:

1. Add a coefficient cfbackg for neutral background source psorbgg to help relieve the numerical difficulty when ng is getting close to ngbackg, especially for impurity neutral continuity equations in more detached plasmas. Using cfbackg[igsp] = 1.0 is equivalent to default. Please use 10 or 100 which can help a lot sometime.

2. Add vacuum neutral model (VNM) to consider neutral bypassing through the vaccum ('void') region, from divertor regions to upstream, based on a DEGAS2 pre-calculated tele-transport matrix. The detailed instructions for using this model will be available soon. Please reach out to Andreas Holm or myself (zhao17@llnl) if you have questions.

3. Three options for computing out-going neutral fluxes for the above VNM are implemented:

   1. based on local half-Maxwellian assumption
   2. CX-diffusion assumption with local approximation
   3. combining the two based on local mean free path and density scale length
      The Option 2 and 3 are based on the advanced neutral model developed by the KU Leuven group.