Restrict genome coordinates

[hyanwong]

It's relatively common to want to restrict the genome coordinate space to show e.g. positions 1000...2000. One way we do this in the draw_svg() command is to look to see if the first and last trees are "empty" (i.e. have num_edges==0), and if so, change the X-coordinates to go from the left of the first non-empty tree to the right of the last non-empty tree. Coupled with the change in #69, that means it's possible to do

ts_arg = ts_arg.keep_intervals([[1000, 2000]], simplify=False)
# Now plot, without showing all the unreferenced nodes

The nice thing about this is that it only shows the "useful" sections of the tree sequence (e.g. in inferred tree sequences we often trim the flanking regions away, which cover telomeres etc).

An alternative, which we also have in draw_svg, is an x_lim parameter (under the hood, I think this simply does keep_intervals(..., simplify=False) before displaying).