Null-check not implemented for penalized likelihood

[stephens999]

The nullcheck flag checks the likelihood achieved by EM against the likelihood for the "null" (all mass on the component with smallest variance). When we use a penalized likelihood (eg method="fdr") it seems we'd like to check the penalized likelihood achieved by EM against the penalized likelihood for the null. The current behaviour might cause ash to be less conservative than we would like for nearly null data.