Make partitions according to a specified number of events.

[lgray]

As discussed in 23 Friday Coffea meeting:

• A spark partition per basket is probably not the best way forward
• Similarly a spark partition per file probably has too much skew.

It would be reasonable to implement a partitioning that tries to fill a set number of events (like 200k) per partition, to avoid later skew and start off with a reasonable parallelization of the processing task.

[PerilousApricot]

ty for writing this down so it's on "the list" -- I'll want #47 implemented first before any more config options get added. I don't want the situation where the only way to find out about options and what they do is by reading the source

[lgray]

Here's the code changes the implement splitting by event:
lgray@989e826

I'll keep it up to date with master.