cf. https://github.com/jacomyal/sigma.js/issues/340

various experiments around gexf parsing: take into account declared atts, trace comments for parseCustom, declarable @cluster_index equivalent, and separate function in sigma_tools for using stock gexf parser (unconnected now)

flags: systematize handling (set in initial setup, cancelSelection, greyEverything, used as params in rendering), rationalize forceLabel, rename customAttrs.neighbors to highlight ; cluster highlight: on click in legend, highlight like neighbors but once ; deselect: background click to deselect is optional

(precomputing once common color/size properties for FillGraph and add1Elem -- and therefore for rendering)

cf. github.com/ISCPIF/gargantext/commit/2e4037c