prepare deps for d3v6

[Fil]

• [x] add deterministic prng in the forceCollide "jiggle" (#160)
• [x] only start the simulation when someone is listening (fixes #45)
• [x] make forceCenter less aggressive (fixes #81)
• [x] tests (#17)
• [x] add forceRadial x, y and angle accessors? (#167)

Note: because of the breaking changes in #81 (which impacts #17), #167 (and partly #45) this branch is "maximalist", and should be published as d3-force@3

The alternative option is to have a minimalist PR with almost no code change (ie only https://github.com/d3/d3-force/pull/171/commits/f429ab1582d397a5c6bad45a6c5e4bde91882a07 and #160), that we can keep as v2.

[Fil]

Split this PR into #172 (minimalist, d3-force@2) and #173 (includes breaking changes, d3-force@3)