Redo MultipleInputFeatureRequirement toward Merge Options

[jmchilton]

This was the first attempt at multi-input scattering - ae11f56 - now that I'm staring at count-lines 6 and 7 it is clear to me that replacing CollectionInfo with a new type of lets say tracker of scattering and input collections is wrong. We should use CollectionInfo and all of its utilities for tracking - we should just feed it "ephemeral" collections created just for tool execution. This has some distinct advantages over the first approach:

• We can track these collections in the DB at some point for tracking the structure of what we did.
• We can build arbitrary ephemeral collections in response to scatter input expressions. e.g. scatter-valuefrom-wf1.
• We can handle non-scattering operations with Ephemeral collections as well (such as supplying multiple lists to a single list input in a merged fashion) - e.g. count-lines7.