Unit checking in the new front end is slow

[Francesco Casella]

The unit-checking feature is the performance bottleneck of the new front-end, even for models that actually don't have unit information in their variables (see, e.g. the AdvectionReaction models of the ScalableTestSuite).

This may not be a big problem, because unit testing usually makes sense when developing new models, and that is normally done using small scale test models, so making this module faster is really not a high priority.

I would suggest that @perost changes the default behaviour to turn off unit checking by default on the new front end, so that we see the performance of stuff that reall matter, and then @mahge930 and @arun3688 have a closer look at performance issues and try to improve them, if this is not too hard.

@mahge930, please make sure there are test cases in the testsuite where unit checking is switched on after the default is changed.

[Per Östlund]

I had Martin disable the unit checking for the Hudson library testing job for the new frontend, but now that you mention it it makes more sense to just turn it off in the new frontend instead. I will do that, it's just a matter of flipping the debug flag to be false by default.