TODO list for apply noise feature

[schweitzpgi]

These are issues to be worked on:

add error message #2635 (comment) -- #2658 fixes C++. Still need to add error message for Python
unify definitions to 1 place #2635 (comment)
deal with multiple defns #2635 (comment) added Ben's suggesting of hashing a name for the key value
remove amplitude damping? #2635 (comment)
phase flip channel? #2635 (comment)