reimplement parameter distribution

Overall goals

• Restore outcommented examples
• Support Monte-Carlo integration over all parameters [Redmine697]. -> #539
• Improve examples [Redmine1014].
• Support arbitrary functions (random variable -> model parameters); requires alternative Py bindings (#232 (closed)).

Immediate steps:

• rm m_beam and m_axis from *Sim (!1454 (merged))
• ready to go for Par-based generic mechanism?
• apply the same to SpecSim