Weighted models, new dataset, new encoding features

Summary of larger updates:

• Added support for estimating weighted regressions
• Added and improved documentation for new datasets: yogurt, cars_china, cars_us
• Exported the dummyCode() function for automatically creating dummy-coded variables in a data frame.
• Added support for auto dummy-coding categorical variables prior to model estimation
• Major overhaul of documentation using {pkgdown}

Summary of smaller updates:

• Changed license to MIT (after doing a bit of reading up on this)
• Fixed dimension-matching issue with user-provided draws for mixed logit models
• Fixed bug in modelInputs where obsID was not a vector for tibble inputs
• Added placeholder hex sticker