WIP: Add passive scalar transport to compressible flow solver

Using the efforts of Aiding this MR adds the ability to run passive scalars in the compressible flow solver. This is currently working on the master branch with the LDG explicit solver. However to complete the MR we should perhaps see if the IP method is also working. A test has been reactivated including two passive scalars.

Note this is an extension of the work by Aidyn Aitzhan (Pittsburgh)