Add a style property to Artist base #1

Create a style property in the artist.Artist class. This will be the first step in having a generic 'style' class which will dictate the appearance of all artists.
To create the style property, it may be necessary to create a style descriptor (to enforce type checking etc..). This is so if more than one style property is needed in derived artists (e.g markerStyle in line2D) code does not need to be repeated