Improve datamodel dictionary 'update' handling of recursion

[dmggh]

The dictionary function ‘update’ for datamodels does not elegantly handle recursion. A brute force implementation can be used as a workaround, but the goal is to implement an override for the function that will recursively apply ‘update’ to objects within a tree. The use case is to update all the metadata of a file (under the key ‘meta’), which is a dictionary that may contain sub dictionaries (hence the recursion). This problem was discovered while creating unit tests for the dark current step.