Implemented the functionalities for sphere rendering
- implemented a class that is the superclass for different types of quadrics
- implemented the sphere class
- implemented the parser functionality that allows to parse a sphere tag
- implemented the functionality that allows to add a sphere to an object
- implemented the functionality that allows sphere rendering
Extended the xml parser
- implemented the xml parser in order to be able to get info about
  sphere quadric properties, like draw style, normals type, face
  orientation and texture mapping type.