Develop XMIAdaptor code

[jasonvan]

https://github.com/jasonvan/prexel/blob/master/xml/adaptor.py - This class is responsible for the conversion of a Diagram object into XMI

• [x] Determine class requirements
• [x] Create class
• [x] Create tests
• [x] Write code to take diagram object and create XMI
• [x] Write code to output XMI to file
• [x] Add aggregation to XMIEncoder

[jasonvan]

See current commit of XMIAdaptor class (test class here). This class is responsible for generating a XMI fragment from a Diagram object. A Diagram is a model that is used throughout the plugin to transmit data about a PREXEL diagram.

XMIAdaptor is not complete as more methods are needed to properly generate the XML tags required for basic XMI documents. Currently, enough methods exist to generate an XMI document that models a class with method and fields. Adding inheritance and aggregation is the next step.

[jasonvan]

Code from #32 was merged into master. This code is responsible for generating XML nodes for XMI tags. This will be used by other classes in the XML module to generate XMI fragments from Diagram objects

[jasonvan]

Commit made that adds XMI generation for basic class and inheritance diagrams

https://github.com/jasonvan/prexel/commit/5a1c08fe286ea589a976041aea110e35f054049f

[jasonvan]

Commit made that adds aggregation including multiplicity

https://github.com/jasonvan/prexel/commit/ad40eee0ecd73c1be9e0f44722fc8378000f651f

[jasonvan]

Work completed in this PR #37