Implemented "XmlElementHelper" to abstract attribute logic

### Background
There is an on-going undo/redo work on a separate branch that implements "ModelBase.Serialize" and "Deserialize" methods. In these methods (and all the derived methods), there is extensive use of attributes in XmlElement. Without this helper class to abstract out most of the identical codes to deal with data-type conversion, the caller codes quickly becomes hard to maintain.

### Side Notes
* This commit is made to remove bulk of the code changes that will eventually go into the undo work.
* All test cases in "DynamoCoreTests.dll" are passing (the class isn't used anywhere else)