absorb collectors into readers

[TaiSakuma]

related to the comment at https://github.com/alphatwirl/alphatwirl-interface/pull/7#issuecomment-369357995

this will be surgical changes.

it will help to solve #10.

steps:

• add the operator + (or method merge()) to Reader
• create an alternative to EventsInDatasetReader, in which readers for the same data set are merged at the end. - EventDatasetReader
• add merge() to ReaderComposite
• add merge() to AllwCount, AnywCount, and NotwCount
• replace the return in end() in EventDatasetReader .
• deprecate EventsInDatasetReader

[TaiSakuma]

• add collect() to ReaderComposite f8b8f92
• add option collector and method collect() to Reader 9be4939

[TaiSakuma]

The collectors are actually doing two things

1. collect results for multiple data sets
2. collect results from multiple jobs for the same data set

The original purpose of the collectors was 1. In early versions of alphatwirl, there only was one job for one data set. The 2 wasn't necessary. The collectors started doing 2 as well when it became possible to split a job for one data set.

It is not easy to include 1 into readers because readers are unaware of data sets.

It is relatively easy to include 2 into readers.

[TaiSakuma]

• add collect() to AllwCount, AnywCount, NotwCount 2f1ccdf