scipipe/scipipe: v0.6: All external dependencies removed

This release mostly contains a lot of small clean-ups and fixes, including a bugfix (#51), but a larger thing is this:

We removed all external dependencies outside of the Go standard library. This is to make it as easy as possible to use Go's vendoring support, when depending on scipipe, so you can - despiite Go's lack in terms of a package manager - can easily fix a specific version of scipipe and include it in your own code and source code repository, for maximum reproducibility (SciPipe is only a little more than 1k lines of code, so should not be a big deal).