multi_update

[eagmon]

This PR corrects a pretty serious oversight that I discovered when migrating wcecoli. If a process has multiple ports that share a variable name, and those ports are plugged into the same store through a composite, then the process could be updating the variable multiple times in a single update. Previously, inverse_topology in experiment used deep_merge, which would merge those updates, arbitrarily overwriting one with the other. We needed the option for both updates to be applied.

To fix this, inverse_topology now uses a function called deep_merge_multi_update. This is like deep_merge, but if a value is already present under a given key, it gets added to a list under the special key '_multi_update'. The apply_update method looks for this special key, and will apply all updates under it to the same store.