fix all nans in GrangerAnalyzer

[Xunius]

This is to fix the issue #173.

GrangerAnalyzer put its outputs in the upper triangle only, which
would be removed by drawmatrix_channels() which takes only the lower
triangle, creating an array of all nans.

This fix changes the drawmatrix_channels() to let it take the upper
triangle instead, then transpose the matrix before plotting.

Another possible fix is to transposes outputs of GrangerAnalyzer into the
lower triangle. However this would break with the old behavior including
failing unit tests (test_granger.py).

Another reason to change the drawmatrix_channels() instead is that
other similar functions, including CorrelationAnalyzer.xcorr,
CoherenceAnalyzer.coherence all computes the upper triangle first,
then transpose to fill the lower, e.g.

idx = tril_indices(tseries_length, -1)
xcorr[idx[0], idx[1], ...] = xcorr[idx[1], idx[0], ...]

But granger is a bit different in that it is not symmetrical so we don't
just mirror the upper triangle into the low.

Another reason to keep the upper triangle in granger is that the
indices of pairs argument (ij) of the GrangerAnalyzer.__init__()
translates more naturally to the upper triangle indices, e.g. pair
(0, 1) -> (row-0, column-1),
(2, 4) -> (row-2, column-4).
If feels more intuitive (to me at least) to preserve this, rather than doing a
transpose here. This also helps pass the test_granger.py unit test.

Related changes needed:

in doc/examples/multi_taper_coh.py, I have to change the nested for loop
so the results are put into the upper triangle. The old for loop:

for i in range(nseq):
    for j in range(i):

New:

for i in range(nseq):
    for j in range(i, nseq):

This is also consistent with CoherenceAnalyzer etc..

I've tested all scripts in the doc/examples folder that involve the
drawmatrix_channels() function. All figures look good.

[arokem]

This all makes sense to me!