Specify and improve AtomicMultiChannelPT duration behaviour

[shumpohl]

Currently the keyword argument duration may be an expression, a boolean or None.

I would like to prepare the changes drafted in #535 which unfortunatly require the more complex changes from #552 by deprecating the boolean argument and only failing in __init__ if we can prove that the durations are incompatible.

Another option would be to be more permissive and only warn in that case that create_program will fail.

Current plan:

• Deprecate boolean duration and treat it as None
• Remove all duration checks in __init__ for now.

[shumpohl]

Implemented in #603