Unit checking in the new front end is slow

[Francesco Casella]

Even after the fixes of #4615, 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.