Rename `GrayscaleImage` and `RGBAImage` about pluto

[Vici37]

@sanks64 I would have a slight preference for the latter suggestion (ImageGA and ImageRGBA) so that the context that these class names composed entirely of capital letters is still there :)

That being said, I introduced the Grayscale image with the intention of providing a lot more methods for data extraction, which we then decided would fit better in its own shard. As far as I know I was the only consumer of the Grayscale image - would it make sense that it should be removed from pluto then? I don't have strong opinions one way or the other on this matter, just bringing it up as on option.

[snacks02]

I'm actually using GrayscaleImage now, so I'd keep it

[Vici37]

Oh, cool! Carry on then 👍

[snacks02]

@Vici37 What do you think?

[snacks02]

Alternatively:

• GrayscaleImage -> ImageGA (GrayscaleAlpha)
• RGBAImage -> ImageRGBA (RedGreenBlueAlpha)