Changed the specification of TrucatedMvNormal

Now alpha = \Psi(b)-\Psi(a) is calculated directly in the definition of
type. Also, cdf(TruncatedMvNormal, x) takes an argument x because now
it really is the cdc of the multivariate distribution.
Fixed cdf call

To reflect change in TruncatedMvNormal and new definition of cdf.
Deleted