fix wrong confidence interval shape in mt_coherence()

[Xunius]

In multitaper.py, the mt_coherence() function assigned an empty array for the desired confidence interval using mt.empty(nf, 2) at line 600. This gave an 1d complex array of length nf, because the argument 2 is passed to the complex=False kwarg of the mt.empty() method.

The correct call should be mt.empty((nf, 2))

Related to issue https://github.com/krischer/mtspec/issues/32