#12 Model Builder block validation

[Anna Wojtowicz]

When creating an input file using the MOOSE Model Builder, look into ways it could be made "smart" by validating the problem setup. For example, no problem can run without an Executioner block defined, so if the Executioner block isn't set, maybe throw some warning that you doin' it wrong.

This may/may not be very difficult to implement, as the YAML file does not specify which blocks are required, only which parameters.