Fix bug in frequency mask handling for PE

By default, the baseline frequency mask gets set to None at some point. In PE, the code checks whether the attribute is set. If so it applies the frequency mask. However, even when not setting the frequency mask manually, it defaults to None. Hence, the baseline has the attribute, i.e. self.frequency_mask=None, and therefore, PE is run with None as frequency mask. None of the frequencies are then considered in the likelihood, which gives rise to errors. Easy fix is checking whether the frequency mask is None in the if statement.

This addresses issue #103 (closed).