Issue with compute_1d_mutual_info

[apeyrache]

in compute_1d_mutual_info, the average firing rate is computed as
fr = np.sum(fx * occupancy, 0)

but this is not the right definition and could lead to erroneous information values.
This should be the firing rate over the epoch, which is not the same as recomputed it from the tuning curves and the occupancy since the tuning curves are not necessarily computed on the same epoch.

We will work on solving this issue.