Only check if model supports mass balance checks if mass balance feature is enabled

[aaraney]

Currently, ngen will throw an exception if any model in the model stack does not support mass balance checks regardless of if mass balance checking is configured. The fixed The new behavior checks if a bmi model supports mass balance checks only if the mass balance checking feature is enabled.

Revisiting this. The exception I was getting from ngen was not related to the mass balance checker.

I do think we should still change the behavior to only check if a model is capable of mass balance checks if configured. Otherwise, stderr is spammed with messages like the following for each divide w/ a model that does not support mass balance checks:

Error(Integration)::mass_balance: Error getting mass balance values for module 'Simple Logical Tautology Handler (SLoTH) Model': GetValuePtr called for unknown variable: ngen::mass_in (/home/user/ngen/extern/sloth/src/sloth.cpp:114)