let mumax3-convert convert wildcards if needed

[JeroenMulkers]

Let Mumax3-convert expand wildcards which are not expanded by the shell.
(pointing a finger at cmd.exe)

[godsic]

Have you tested on both Linux and Windows?

[JeroenMulkers]

I do not have a Windows machine, but I cross compiled mumax3-convert for Windows and tested in Wine.

I also tested the modification on linux. Following commands, give the same output:

mumax3-convert -png *.ovf
mumax3-convert -png "*.ovf"

[gen-xu]

I have tested wildcard on Windows 10 and it works.
Another comment: is it possible to use cuda to do the convert?

[JeroenMulkers]

The gpu can't read or write files. So I doubt that using cuda in mumax3-convert will speed up the conversion much, if any at all.