Ship gnuplot with Windows builds.

[godsic]

@JeroenMulkers @JLeliaert

Guys, so many people complain about problems with gnuplot, in particular, on Windows. I feel like setting up gnuplot is way too hard for prospective users. Shell we consider providing it ourselves and make mumax3 look for it in its binaries folder?

[JeroenMulkers]

That would indeed be a good solution for these complaints.

Another option is to remove the gnuplot dependency and use the gonum/plot package to make the plot. Let me give it a shot. If it does not work very well, I will add gnuplot to the binaries folder as you suggested.

[JeroenMulkers]

In pull request #272 , I used the gonum/plot package to make the plot. I think that it works quite well.

@godsic @JLeliaert , what do you think? Do you have any objections against the usage of the gonum/plot package?