Implement Lognormal with option for lower level of quantification #280

If a measurement is below the lower level of quantification (LLQ), the true value of the measurement is unknown. All that is known with certainty is that the measurement value is between 0 and the LLQ. In principle, different model assumptions could be made about the likelihood of different measurement values in the 0 and LLQ, but the simplest assumption is that all measurement values are equally likely. This is the assumption we will go for in our implementation.