Allow default attribute on data parameters.

Like `optional`, but if no value is select for the data parameter a wrapper for a tool-relative file (instead of a dataset) will be fed to Cheetah.

This has been requested for Galaxy tools before, my selfish desire for this is to implement default values on File parameters in CWL tools. https://gist.github.com/jmchilton/d0985f141e4fc486147d

Start by tracing how data parameters are represented and how a tool-relative file wrapper is fed to Cheetah; the issue does not name files or tests. Compare the requested behavior with the linked CWL default-value example, then define the expected behavior for an unselected data parameter before implementing and testing it.

Written by the indexing model from the issue text.