Introduce the concept of pipeline monitoring: A new monitor class is
passed to the pipeline.run() function. The main idea is to separate
the monitoring from the code that builds pipeline. Through the build
process various methods will be called on that object, representing
the different steps and their targets during the build process. This
can be used to fully stream the output of the various stages or just
indicate the start and finish of the individual stages.
This replaces the 'interactive' argument throughout the pipeline
code. The old interactive behavior is replicated via the new
`LogMonitor` class that logs the beginning of stages/assembler,
but also streams all the output of them to stdout.
The non-interactive behavior of not reporting anything is done by
using the `NullMonitor` class, which in turn outputs nothing.
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