Allow touchstone package to be installed from custom source.

[plietar]

trafficstars

Instead of allowing the user to select a particular branch of the lorenzwalthert/touchstone repository, allow an arbitrary package to be used instead. This gives greater flexibility for development in forks of the repository.

[plietar]

Sure, would be happy to take over maintenance if that helps. I don't plan on working on it actively but we have started using it so might as well. If you do decide to transfer it, could you please do it to the reside-ic organisation?

[lorenzwalthert]

@assignUser would you be ok with that transfer or do you think it should live elsewhere? I also considered r-opensci but I think the hurdles for this would be higher...

[lorenzwalthert]

@assignUser can we merge this?