add clustering force

[randName]

Based on what I wrote for https://github.com/randName/ASDvis

Sounds like "Charge Colors" mentioned in #2 , but that seems to be forgotten.
Will document later, but I'm not sure if there will be changes to implementation.

[Nibiri]

@randName Hey, do you have any visualization examples or illustrative examples?
I think this PR could help me solve my problem.

[randName]

yep: https://randna.me/ASDvis/
it keeps the dots within the circle groups

[Nibiri]

@randName cool visualization, do you think it would work if the nesting level was higher?

just like here

[randName]

@Nibiri I'm unsure of how nesting would work, but if you want annular groups then you can probably borrow some stuff from forceRadial

feel free to use the grouping stuff I wrote, that's pretty much the only thing i added on top of forceManyBody

[Nibiri]

@randName Good,thank you for you answer!

[Fil]

In #167 forceRadial has an accessor for the center, which can help clustering around various central nodes. I'm not sure it totally covers the use cases you had in mind, though?