Choose a default for ambiguous formats

[timholy]

save("myfile.ppm", img) used to generate an error, because there are two formats (:PPMBinary and :PPMText) defined. Users will rarely care, and in almost all cases you want :PPMBinary anyway. So I changed the behavior from "throw an error upon creating a new file when the format is ambiguous" to "choose the first one in the list".

I also provided a new mechanism for forcing the format: save(format"PPMBinary", "myfile.ppm", img). You might have thought that save(File(format"PPMBinary", "myfile.ppm"), img) would have been the better choice, but that runs into a problem: unless the (new) Netpbm.jl package has already been manually loaded, it falls back to the "error: no saver defined for ..." method. So we need a method with distinguishable signature, and this seemed to be the obvious one.

[SimonDanisch]

I tried to debug this but seems like it's better if you fix that error.
I guess mapinfo is only defined for Gray{U8} ?
But then I ran into a method error with mybswap(::Gray{U8}), which is where I decided to stop^^

[timholy]

Ah, it's fixed locally but I need to push.

[SimonDanisch]

rest seems to work fine!
I'm definitely in favor of your proposal.
Back when we had that discussion I was just happy that there is already a solution, but this is definitely better =)