PrimaryScatter filter and phsp attribute

[dsarrut]

Allow to store PrimaryScatterFlag (or to filter) test077

doc TODO

[dsarrut]

WARNING : phase space actor policy changed. Now there are options:

• store_entering_steps (True by default)
• store_exiting_steps (False by default)
• store_first_step (False by default)

NOW -> phsp.steps_to_store = "entering exiting first"

[dsarrut]

Merged in #403 (do not merge in master yet)