New simulation functionality

Summary of larger updates:

• Added support for simulating shares for a set of alternatives given an estimated model: simulateShares(). This is similar to the predict() function in mlogit.
• Removed support for using an estimated preference space model as an input in the options() function. I found this just far too confusing, and instead encourage users to supply a WTP space model with the computed WTP from a preference space model as starting parameters.

Summary of smaller updates:

• Updated the summary() and main logitr() functions to keep the basic information (run #, log-likelihood value, number of iterations, and output status) whenever numMultistarts > 1. Previously this information was only kept if keepAllRuns was set to TRUE.