Custom path to reference results

[marcusfuchs]

For #393 :

• Add option to specify a custom path for reference results so that we can also compare results to references outside the tested library
• If no custom path is given, the standard way of searching for/creating the reference result directory should behave as before.

[marcusfuchs]

@mwetter After using this for my use case it seems like the changes already solve my original issue. If you would like to collaborate on you additional requirements mentioned in #393 I'm happy to help if this is possible without being too familiar with applying this for different tools.