Add downsampling to select_sketch in rust

[luizirber]

Currently sig.select_sketch in Rust only return sketches that match exactly a template. But... If there is a scaled=10 sketch and the template is scaled=100, the sketch can be downsampled.

There is some code to do that in https://github.com/luizirber/2021-01-27-olga-remove-protein/commit/e9603f36e8e57488a490767056a44f071af1026c, but this triggers copies all the time. Might need to think better (maybe an extra parameter, downsample?) on how to control this behavior.