Enable explicit tool creation of collections with static or determiniable number of datasets.

By "static" I mean tools such as a FASTQ de-interlacer that would produce a "paired" collection with two datasets everytime. By "determinable" I mean tools that perform N->N operations within the same job - such as a tool that needs to normalize a bunch of datasets all at once and not in separate jobs. (For N->N collection operations that should or can be done in N separate jobs tool authors should just write tools that operate over a dataset and produce a dataset and let the end-user 'map over' that operation.)

There are still large classes of operations where the structure of the output collection cannot be pre-determined - such as splitting files (e.g. bam files by read group) - that are not implemented in this commit.

Model:

The models have been updated to do a more thorough job of tracking collection outputs. Jobs just producing HistoryDatasetCollectionAssociations works fine for simple jobs producing collections - but you don't want to map a list over a tool that produces a pair and produce a bunch of pairs HDCAs and a list:pair HDCA- you just want a bunch of pieces and the one list:pair at that the top.

Workflow:

Workflows containing such operations can be executed - but the workflow editor has not been updated to handle this complexity (and it will require a significant overhaul) so such tools are not available in the workflow editor.

Tool Testing (WIP):

TODO:

Tests:

Includes two tools to test this - one that uses explicit pair output names and one that iterates over the structure of input list to produce an output list.

Includes several new tools API tests that test the tools described above via the API and implicit mapping over such tools. Includes two new workflow API tests - one that verifies a simple workflow with output collections works and one that verifies mapping over workflow steps in collections works.
Prevent tools producing collections from showing in workflow editor.

Certainly feasible someday but workflow editor logic will require significant overhaul.