Unrooted layout fix

[brj1]

Hi Guangchuang,

I was trying to use the unrooted layouts in ggtree and I found that my trees always came out looking squished. Looking in the code I noticed that in ggtree function a yscale is added to the plot expanding the yscale by 0.6, which is more than my tree's length. Hence, I changed the ggtree function to use "coord_fixed` instead to plot with an 1:1 aspect ratio, which I think make more sense as a default.

I also wrote in ape's unrooted layout. Though I didn't make it work with variable branch.lengths yet.

I put these changes in separate commits in case you don't want the ape layout.

[brj1]

I decided to change the ape unrooted layout to call the ape::unrooted.xy instead of using its code. I need to test it before I reopen this PR.

[brj1]

I've changed the ape unrooted layout I added to directly call ape::unrooted.xy to ensure that the x, y coordiantes are the same as in ape.

[GuangchuangYu]

thanks.