transparency for pov ray renderer

Hi,

I added a keyword "transmittances" to the POV class constructor (may change it to "transparencies" or something similar) which allows to render also partially transparent atoms using pov ray. I am not sure if there is much interest in this feature but I needed it for some of my figures. Moreover, I do not know if the implementation (rather quick'n'dirty) meets ase standards, hence "work in progress". One may as well think about defining a "material" macro in the respective pov files, similar to what VMD does. This should then combine color, transmittance, texture,... I there is any interest along these lines, I'll see what I can do.

Happy to hear your comments.