Bounding box improvements and some fixes.

SMTK's widget for editing attributes visually with a bounding box will now accept a variety of formats for specifying boxes (a 6-vector of bounds, two 3-vectors of minimum and maximum points, two 3-vectors describing a center point and half-lengths, and an optional 3-vector specifying rotation angles).

This should address #229 (closed) by replacing qtDescriptivePhraseEditor with a QLineEdit.

This MR also adds a CODEOWNERS file that gitlab can use to suggest reviewers for merge requests.

Finally, it fixes an issue (required for the bounding box changes) with the attribute GroupItem class where find(childName) would cause an assertion if childName was not present (rather than just returning -1 which it was clearly designed to do).