HANK: add a routine to check the steady-state input

Add functions and options to validate steady-state input for heterogeneous-agent models in the matlab/+hank folder. Each function includes proper documentation headers (INPUTS/OUTPUTS) and error messages are standardized for consistency. A pretty thorough test test is implemented in tests/hank/ks.mod. This creates a basis for more robust handling of steady-state inputs in heterogenous-agent models. It has no impact on existing modules.