Multiple field outputs for MultiSpec solver.

[airaudofacundo]

Pressure, mass fractions (Y_k) and mach number as field outputs for the MultiSpec solver. Ideally Mach number would be implemented to other solvers as well.

• Mass fractions (Y_k)
• Temperature gradient / heat flux
• Pressure
• Mach number

[adityakpandare]

Temperature gradient and/or heat flux at the walls are also needed as field outputs for the multispecies solver. Adding that to the list.