GhPython mesh artist issues with color handling

[gonzalocasas]

The GH python mesh artist makes too many assumptions about color handling, specifically, there are (at least two issues):

• If a color is specified, it is converted to rgb255 when creating the object, which discards the alpha value of the color. This could be fixed by changing all rgb255 instances to rgba255
• It seems impossible to not define a color and get Rhino's default color. This might actually be intentional, but it's a bit of a questionable default behavior.