Update type of Transformations._sync_kwargs to be containers/sequences

[jdkent]

ref #835

sync_kwargs should be a set instead of a boolean to indicate which arguments should be synced.

[effigies]

Just updating this so anybody looking will understand it:

In #836, a _sync_kwargs class variable was added to transformations. Here it's proposed to change the type to some sequence/container type to allow some arguments to be synced and others left un-synced.