Support multiple metric arguments per stage.

Support multiple metric arguments per https://groups.google.com/forum/?fromgroups=#!topic/nipy-user/gvKCXDlSQDU, discussion after Jun 3.

Note that the pull source is the https://github.com/FredLoney/nipype ants_multiple_metrics_per_stage branch, not the master branch.

The change is tested with the new reg5 doctest.

The sigma unit suggestion in the cited forum topic prior to Jun 3 is not included in this pull request and will be submitted as a separate pull request.