FileSelector defaults to the first globbed path

[maximlt]

Addresses the first issue raised #545, i.e. that the FileSelector parameter is not updating its default value to the first path found in objects, which is inconsistent with how Selector works.

[jlstevens]

Looks good to me!

Right now it looks like the glob is run when the parameter is declared which is probably what I would expect. Unlike Selector, the set of 'objects' is implicit so I think it might be worth stating in the docstring that the glob is run at the point where the parameter is declared.

[maximlt]

Thanks for the review! I'll update the docstring in another PR.