Query overhaul

[pranavtbhat]

Move away from macro based piped queries. A clean composable syntax would probably work much better.

New queries:

• [ ] eachvertex @query g |> eachvertex(A=v.p1, B=v.p2 + 3, C=v.p4) should return a dataframe.
• [ ] mutator queries @query g |> eachvertex(v.p1 = v.p2 ^ 2) should run a setvprop on the result of the computation`.
• [ ] sort on columns
• [ ] group by on columns
• [ ] Motif matching for edges

The ability to override getfield would definitely come in handy.

Something like:

immutable Vertex
end

immutable Edge
end

const v = Vertex()
const e = Edge()

getfield(::Vertex, s::Symbol) = VertexProperty(s)
getfield(::Edge, s::Symbol) = EdgeProperty(s)

The queries can then look like:

filter(g, v.p1 < 5)
eachedge(g, e.p1, e.p2)