Add warning if prior allows for negative variances and if negative estimated variances are encountered. Closes #522

Due to the use of variances for Sigma_e and subsequently backing out the standard deviation from these variances, the sign of the standard deviation does not matter and no bound needs to be imposed.
Merge pull request #609 from JohannesPfeifer/warning_lower_bound

Add warning if prior allows for negative variances and if negative estim...