Consolidate ls and listVariable

[dehann]

all the list functions should support a common permutation set of arguments with which to filter the results.

ls(dfg::G, node::Union{InferenceVariable, FunctorInferenceType})

# and
listVariables(dfg::G, regexFilter::Union{Nothing, Regex}=nothing; tags::Vector{Symbol}=Symbol[], solvable::Int=0)

List of desired filters

• regex
• Type (Variable or Factor)
• solvable
• tags

[dehann]

Should a separate filterDFG verb be added, then that function can still be used inside the list functions for user convenience. Dispatch provides plenty of room for flexibility in the list verb API.

[Affie]

xref #1085