Decision/Vote: go with state for both variables and factors.

Ticket Acceptance Criteria:

• data is hard to use, what is better: ?
• must work for Variables and Factors
• must have singular and plural
  • variable has solveKey superposition, therefore satellite nodes. Not so for factor.
• should this indicate mutable or immutable somehow? Chances are this is definitely mutable
• define why is this different from metadata
  • metadata is used by humans and usually set final before any solving or compute operations are performed.
  • this data_field has to do with compute or algorithm mutations that the novice human generally does not mutate.

Currently we have the following nouns referring to solver data:

VariableNodeData - Struct
PackedVariableNodeData - Struct
GenericFunctionNodeData - Struct
FunctionNodeData = GenericFunctionNodeData
data - field in FactorDFG
solverData - field in FactorCompute
solverDataDict - field in VariableCompute
solverData - field in VariableDFG
FactorData = PackedFunctionNodeData{InferenceType}
SolverData - Retrieve solver data structure stored in a variable/factor.
VariableSolverData - for get/add etc.

FactorOperationalMemory is also partially in the mix here.

Related:
The observation function is currently part of the factor solver data, see #1119 and #992