sample() method updated to accept user-defined upper bound on variances

[devindg]

• 'upper_var_limit' and 'max_samp_iter_factor' have been added as arguments to sample() method. These control, respectively, the acceptable upper bound on variance draws and the maximum number of sampling iterations before an exception is raised
• Added attribute 'high_posterior_variance' to BayesianUnobservedComponents class that summarizes the amount, if any, of high variances sampled, where "high" is any variance that exceeds the sample variance of the response variable.
• Reworked the if/else logic in sample() method for accepting/rejecting samples. It's fundamentally the same as before.
• Created custom exception for maximum sampling iterations