odgi draw: add node sparsification factor for SVG output

[AndreaGuarracino]

This allows us to make smaller SVG output for big graphs

-f 0.1

-f 0.3

-f 0.5

-f 0.7

-f 0.9

-f 1.0 (no node sparsification)

[ekg]

A related trick would be to make the nodes proportionally longer, filling in the gaps.

[AndreaGuarracino]

Machiavellian. Not so clean with strong sparsification (layouts with stretched nodes to the left):

-f 0.1

-f 0.3

-f 0.5

-f 0.7

-f 0.9

[ekg]

I wonder if the sparsification could be driven to remove short nodes first. That might retain the structure while greatly reducing the number of nodes.