Introduce DSCoreNodes.DSColor class

Note that this is just a skeleton class to merge into **ChocoButter** branch (note that most of the **DSColor** codes are disabled by a preprocessor). This commit lays the groundwork for **DSColor** work in [**Migration branch**](https://github.com/Benglin/Dynamo/tree/Migration) so that no one will attempt to port **dynColors.cs** in **ChocoButter** without knowing that the class has already been ported in **Migration** branch.