Add arguments to set bounds when visualising sample paths

[szhan]

When the number of data points get too large, bokeh gets slow, so it would be useful to have arguments to bound parent node id and/or genomic position/site index when calling create_sample_path_vis_app.

I think the following argument should suffice:

• range_nodes=(min, max)
• range_position=(min, max)

Default should be the full ranges.

[szhan]

I'll leave implementing range_site_ids=(min, max) as a separate issue.