Add hysteresis to writing state

[GDYendell]

Currently the FP.writing summary attribute just does any of FP.X.writing, which is not what we want when we are starting. We need to know that they have all started.

Manually define FP.writing in initialise to override the automatically generated attribute and pass as the accululator function an instance method that stores the previous state. Then call anyif we are currently writing andall` if we are currently not writing.

Acceptance Criteria

• FP.writing only goes high when all are true, and only goes low when all are false