Transport source must include zeros (or other values) when two tracers are considered (seen on Ubuntu machines only)

To run the transport demo of the short course we had to add a source of zeros for tracer2, when we had a tracer1 in the rainwater so that it could be run in the Ubuntu container. Development on Mac was not throwing out error. Test example below:

https://github.com/amanzi/ats-short-course/blob/ats-short-course-20250908/03_transport/.transport_solution.xml