[execution_dependencies] conditional execution [feature request]

[casperdcl]

related to #1193 -> #1213

add options to only execute dependents:

• once per kernel execution
• if modified since they were last executed

[Alexis-Prel]

Hi, just a comment : I believe this is a great idea but should be on a cell-by-cell basis (for example by applying a lazy tag on it). Consider the following examples:

a += 1

import time
t = time.time()

The user might or might not want to re-run these every time regardless of the cell modification status.