Add PublisherEvery

[smaldini]

Signatures:

• PublisherBase<T> : <T> PublisherBase#every(int)
• PublisherBase<T> : <T> PublisherBase#everyFirst(int)

Return the last or the first element of the given batch count. Allows for simple sampling with fixed size.

[smaldini]

Closing issue, fixed by window().flatMap(PublisherBase::last) and window().flatMap(PublisherBase::next) and