first diagonal NaNs for smoothed expected_cis

[gfudenberg]

the first two diagonals of a contact map are often avoided for technical reasons. Default smoothing, however, does not propagate this to the .smoothed columns. We currently manually undo this in documentation, but this should probably be done by expected_cis by default (and then notebooks can be updated).

A previous (un-merged) PR had suggested:

cooltools/cooltools/api/expected.py

Line 1047 in 1e953e3

# propagate nan

, but this would need to be modified after the API updates!