Hotfix image filename validation

[neutrinoceros]

PR Summary

The following script has two bugs

from yt.testing import fake_amr_ds
from yt import SlicePlot

ds = fake_amr_ds()
p = SlicePlot(ds, "z", "Density")
p.save("1.2.3")

1 - the logger contains a stupid entry (not meaning offense to anyone but myself, I authored it)

yt : [WARNING  ] ... Ignoring supplied suffix '.png' in favour of '.png'

2 - the resulting file is named 1.2.png, while it should clearly be 1.2.3.png (discovered this while trying to embed matplotlib version numbers in file names...)

This fixes both these problems and provides better testing (yay, pytest !) for the culprit function.

[cphyc]

LGTM!

(sorry for closing/reopening, that was a mistake!)

[cphyc]

Why not?

Suggested change

	suffix = ".png"
	suffix = ytcfg.get("yt", "default_image_format")

[neutrinoceros]

I'm ambivalent. I don't like having a parameter with "default" in its name, makes it a bit hard to know what we're talking about. That being said I like the idea of parametrising a "preferred" output format (though that name isn't perfect either). I'd like to think about this a bit more and maybe do it in a separate PR.