WIP: Add ability to assert metadata properties on input dataset parameters.

[jmchilton]

[bgruening]

This is cool! What is needed to get this in? A validator to only show tabular files with 3 columns, or columns with a specific name ... very nice!

[bgruening]

@galaxybot test this

[bernt-matthias]

Yeah, lets revive this ..

[jmchilton]

@bernt-matthias I'm going to close this out - but feel free to rebase and reopen if you're still interested!