Proper runner testing + fixing OpusPocusStep.state consistency

[varisd]

A preparation step for fixing/finishing bash and slurm runner. Includes:

• Added the most critical OpusPocusStep and OpusPocusRunner unit tests. Solves issue #31 for the time being.
• Fixed the OpusPocusStep.state property; now having a proper setter method and returning state value based on the value stored in step.state file
• Simplified the generated step.command file by moving code to dedicated OpusPocusStep methods (called byt step.command)
• Fixed several noqas, mostly typing and exception messages.