Added substitution models that use the ascertainment bias, also know as Lewis correction.
They are invoked like the standard models, with an ASC_ prefix, for instance, -m ASC_GTRGAMMA
or -m ASC_BINGAMMA.

Note that, in some cases numerical problems might occur, it's currently unclear whether this is
an implementation issue or a theoretical issue with the correction. However, it seems to rather be
a theoretical problem.