Numeraire: always print; and enable auxilliary vars

Two issues re the numeraire:

1. It would be good to print the numeraire with the solve when it's user defined. At least when there's only a single fixed variable, and the numeraire is therefore clear. And maybe for multiple fixed variable values, just include them all; they're basically all equivalently the numeraire.
2. I think we should enable for a fixed auxiliary variable to be the numeraire. My testing suggests that it's not functional at this point. Fixing an auxiliary variable still selects the largest consumer as the default numeraire.