Truncated normal variance

There are three changes, all of which help us to compute the moments of a truncated Gaussian.
First, there was a bug in the implementation of the variance for Truncated Normal moments.
Second, the moment calculations often fail because the normalizing constant has a large error.  This has been fixed by making sure that we use ccdf if cdf(d, u) and cdf(d, l) are close to 1.
Third, tests have been added, one of which fails if the second alteration is not made.
Not clear: they really tradeoff computation time against accuracy (because if the test passes then you've got to do more work).  Some people will care more about speed, others will care more about accuracy.