Configs that have been evaluated are now ConcreteConfigs

This way, we don't overwrite the existing config object, which means we can save
the minimal representation even after using the config to start a simulation.
Updated the buttons on the 'Run' tab of the GUI.

Pause and Stop did the same thing, so now there's just Stop. Added a Reset
button which will delete the existing solver object. Buttons which cannot be
used are automatically disabled.
Added Doxygen documentation for Class ConvectionSystemUTW
Fixed units in heat release rate graph label