Various TODO items

[kkoreilly]

Various TODO items that used to be in the README (not mine):

• ch7: fix bg description of da learning --- kinda buried

• ch7: update PVLV to latest bvPVLV version --- use pvlv readme?

• ch8: update theta, add pmn / atn stuff --- in general discussion is a bit disorganized and out-of-date.

• ch10: todo: nback model, hierarchical control model