Add simple solver for ConsGenIncProcess

I've tested it on the PersistentIncomeShock model, and it reproduces the solution exactly. Strangely, it's 5-6x *faster* than the existing code, even though I made this version by "flattening" the old version. That probably means there's a hidden loop of some kind that I (unknowingly) skipped over.

Also: I think PersistentShockType doesn't initialize properly with default parameters. I'll look into it.