Aggregates:
  - Rename Aggregator into Aggregate
  - Export Measures and Dimensions classes as Aggregate.Measures and Aggregate.Dimensions
  - Use assynhronous fetch_all( receiver ) instead of synchronous get()
  - Use Pipelet.subclass() and Pipelet.build()
Pipelet.. _fetch_source(), get(), and fetch_all(), details:
  - Implement _fetch_source( receiver ) to fetch source content without transform
  - Use _fetch_source() to refactor fetch()
  - rename deprecated Pipelet..get() to __get() before final removal
  - fetch_all(): Add exception if a chunk is received after the last chunk was received
Aggregates .Measures and .Dimensions: rename ..fetch() into new _fetch_source() as returned value refers to the source