Report generation likely to generate bad paths on Windows

[taldcroft]

See sot/sparkles#21

[taldcroft]

I looked into the HTML output and source code and templates. For default usage (in particular the way report generation gets called from aca_preview) there is not a problem here. If rootdir is more than one dir deep then problems could happen.

For reference the right way to ensure correct POSIX-ness is to use the to_posix() method on a Path (which might be a WindowsPath).

[taldcroft]

I'm removing this from the 4.4 milestone because the current baseline process for use on Windows does not include proseco acq or guide report generation. The interface to sparkles always uses report_level='none', and there is no strong driver for FOT to be running this. So this is still out there but not a high priority.