Add test signal input architecture to plant model

[gerth2]

For non-serial plant model execution, model inputs are currently just jammed into the sim_init.m file. Create a multi-testcase architecture for pre-defining a number of waveforms for inputs with meaningful testcase names. Dev should be able to easily switch between testcases

[schandrachary]

Can I take up this activity if you don't mind?

[gerth2]

All yours!