major overhaul of inner mechanics of package

[joshyam-k]

• population data is no longer passed to fit_zi() or boot_rep()
• fit_zi() just returns model parameters for a single fitting and boot_rep() returns parameters from each model fitting in the bootstrap in nicely formatted matrices
• full mse estimation now happens in a function called generate_mse() which formats outputs before passing to an optimized c++ function called dom_preds_calc

Still to do:

• catch and remove occasional NA's
• benchmark more thoroughly
• allow new levels (see allow new levels #10)

[joshyam-k]

This PR also renames the main function (unit_zi -> saeczi)

[joshyam-k]

closing for now to fix some issues...