[Developers] remove dependency of NaNChecker from MoL

[Bela Szilagyi]

> For completeness, NaNChecker should provide its full API via aliased
> functions.


One feature I always wished in NaNChecker is to be able to call it at any time 
for any set of variables.  Which, I guess, could be well taken care of by 
adding the aliased function interface.

Another feature I find equally important (with mesh refinement) is to check 
past time-levels as well, if available. 

Personally I ended up writing my own nanchecking routine, looking for all the 
variables I use and on all their timelevels.  Perhaps if we give easy access 
to the functionality of NaNchecker (and extend it a bit), it could become a 
useful debugging tool (rather than being used mainly as an emergency brake, 
as things stand at the moment).