Generate Node: detect rgb attributes and create color primitive attributes

[mistafunk]

e.g. export color.rgb as color[0], color[1], color[2] float attribute this should avoid any awkward "color_dot_r" parameters in e.g. VEX nodes

note: this is not about how the assign node passes rule attributes to the generate node

[mistafunk]

discussing attribute naming in the sidefx forums: https://www.sidefx.com/forum/topic/53124/?page=1#post-238794

[mistafunk]

• implemented conversion framework from materials/reports to houdini primitive attributes
• all float attributes which match the pattern "foo.{r,g,b}" will be converted to houdini foo[3] float tuples

[mistafunk]

merged to master