Accomodate code_source=ppmstar in mppnp output

[fherwig]

• If code_source = PPMstar then we are writing two iso_massf cols, one for downstream and one for upstream. They will be called iso_massf_up and iso_massf_down.
• nugridpy get method needs to read header attribute code_source and, if that is -PPMstar then it must read two iso_massf columns. It must provide for an argument stream with values up, down and average representing either the up- and down-stream as they are written out into the se files, and average returns the average.

master document is in CSA gitlab repo advection-streams - check there for updates