Investigate splitting factor operational part from 'constant' part of factor

[Affie]

If it can be split so that factor solver data only contains the working memory serialization becomes easy as solver data is just ignored and rehydrated when unpacked for use by the solver and there can possibly be no PackedFactor.

• What fields are always constant?
• TODO...

'constant' in this case means not changed by the solver

In other words:
Should we move non FactorOperationalMemory to DFGFactor:
Maybe: fnc, multihypo, nullhypo, inflation ?
that way we split solverData <: FactorOperationalMemory and constants