Add multi-rate gbode test job

[casella]

Currently, we have an experimental on-demand gbode job for LibraryTesting, setting the -d=newInst compiler flag (this could actually be removed) and the -s gbode runtime flag. This runs the default, single-rate GBODE algorithm (I guess it's ESDIRK3, though unfortunately this is not mentioned in the documentation.

We need another experimental on-demand 'gbode_multirate' with the same setting, except that the runtime flags should be

-s gbode -gbratio 0.05

so we can test what happens with the multirate version of GBODE on the entire library testsuite.

@adrpo can you please take care of this? I guess it should be quite easy, just make a copy of the gbode job setup and add -gbratio 0.05 to the simulation flags

[casella]

In fact, duplicate of #102