v0.2.0

Notable changes:

• The class Expression and its alias GlobalVariable: which basically is a function without any input. When added to a MathWorld with a name, its expression can depend on other objects and see its value dynamically updated with respect to them.
• Adding objects to MathWorld returns now an expected<object handle, error> so the method does not throw any longer if the name is already taken or it is of the wrong format.

Full Changelog: v0.1.0...v0.2.0