Smem str constants

[scijones]

The ability to add LTIs via a string alias that persists across add commands. A new table stores the aliases.

[scijones]

The only reason I'm not moving forward immediately is because I have yet to write a unit test. the author has already provided a working example that could be used for such -- it just needs to be put into the unit test framework. Otherwise, I've built the code locally, played a little with it, and it seems to make sense (having read the changes line by line.)

[garfieldnate]

Any documentation changes required as follow-up? The manual, perhaps?