A note on double Henyey-Greenstein phase function

- Explicit solution form for double Henry-Greenstein phase function.
- Removal of abnormal behavior of double Henry-Greenstein (DHG) phase function.
- The improved double Henry-Greenstein phase function is more accurate than HG amd DHG in general.

The abnormal cases for double Henyey-Greenstein phase function are discussed. Based on an explicit analytical solution with a physical restriction, the abnormal results of double Henyey-Greenstein phase function can be removed.