Making the swc_parser more tolerant of different spacing policies

Thanks for your work on SharkViewer! A very useful contribution.

I tried to render a file from neuromorpho.org (included in the pull request) and failed. It turns out each input line started with a space. This was not accepted by your original regex, which was meant, presumably, just to filter out comment lines.

The new regex is a bit more complete, and it accepts flexible spacing.