Add extension method to determine the root element when schemas use 'salami slice' pattern

[mamift]

When XSD's are authored following the 'salami slice' pattern, determining the ultimate root element(s) requires manually inspecting the schema which is annoying and tedious, but brute forcing it is possible with the backward inorder tree traversal algorithm.

Impact: add a CLI argument that also requires the XSD file path and then return the name of the root element(s) in the CLI.