Fix bug 241 - drop color function or radius on zoom

[jeffreylzkong]

1. Math.round() could identify the current drops date as duplicated incorrectly. In the demo data, many commits are very close and considered duplicated after Math.round();

2. drop.enter() takes care the new drops, drop.exit() takes are the removing drops. But current rendered drops are not taken care of, so color and radius are not called.

[tobigit]

I have a custom radius function to set drop dimensions. But when I scroll or zoom the view, the size of each drop changes arbitrary. Does this commit / PR fix the radius-issue?
thank you 👍

[jeffreylzkong]

@tobigit Yes, both custom radius and color function are fixed. My project also need custom functions which must work on every zoom/resize/drag. This fix works perfectly and I believe it can help others as well.

[Busteren]

Just wanted to mention that 2. is fixed in this pr: #259.
For 1. how close does the date need to be, I assume it is with time? (Just curious)