Review verbs for DFG v1.0

[Affie]

Verbs here: https://github.com/JuliaRobotics/DistributedFactorGraphs.jl/wiki/Standard-Definitions-of-API-via-verbs-and-set-theory
With regard to:

• Signature
• Return Type
• Error Behaviour

Verbs:

• list
• get
• add
• update
• delete

TODO add all verbs for DFG v1.0

[Affie]

list
Synchronously list the labels of an existing (composite) data container/object with optional filters.
- Regex filter on label
- Filter on Solvable
- Filter on Tags (tag in variable.tags)

• Synchronous
• Example nouns Variables, Factors, VariableNodeData, PPE -> Vector{Symbol}
• Returns empty list if the container exists but empty
• Throw typed Exeption KeyError or custom exeption if the container does not exist.