Add scaling factor to swc reader

[kyralianaka]

As it turns out that not all swc-like files list x, y, z, and r in um, we talked about adding an optional scaling factor argument to read_swc. This would then likely be one line in swc_to_jaxley that does the conversion of the relevant columns of content. Let me know if this sounds alright to you all and then I can write it and use it.

[michaeldeistler]

Yes I would love to have this.