gstlal-inspiral/python/stats/inspiral_extrinsics.py : leo signal model updates

added more comments to point to Aaron's technical notes.

This also fixes the negative ∑λ2η2\sum\lambda^2\eta^2∑λ2η2 happening to some IMBH templates.

Given the new signal model without KDE, in order to avoid nan values returned, the 2D histogram is not globally normalized to the number of samples, n, but pre-normalized in log scale for each SNR column and let normalize() take care of some numerical error to be normalized in the end. We can ignore n as it won't be KDE-ed anyway.