Implement GraphML for DiGraph

[sequencer]

Contributor Checklist

• [ ] Did you add Scaladoc to every public function/method?
• [ ] Did you update the FIRRTL spec to include every new feature/behavior?
• [x] Did you add at least one test demonstrating the PR?
• [x] Did you delete any extraneous printlns/debugging code?
• [ ] Did you specify the type of improvement?
• [x] Did you state the API impact?
• [x] Did you specify the code generation impact?
• [ ] Did you request a desired merge strategy?
• [ ] Did you add text to be included in the Release Notes for this change?

Type of Improvement

• new feature/API

API Impact

Extends API of DiGraph, if T implements trait of GraphMLVertex, it is can emit a GraphML string with toGraphML

Desired Merge Strategy

Release Notes

Add GraphML generation

Reviewer Checklist (only modified by reviewer)

• [ ] Did you add the appropriate labels?
• [ ] Did you mark the proper milestone (1.2.x, 1.3.0, 1.4.0) ?
• [ ] Did you review?
• [ ] Did you check whether all relevant Contributor checkboxes have been checked?
• [ ] Did you mark as Please Merge?

[sequencer]

Sure, working on it.