BibIndex: refactoring

* Refactoring of BibIndex, implements inveniosoftware#1710:
  `task_run_core` in `bibindex_engine.py` is now easier to
  follow, and the decision of which records need to be reindexed to
  the tokenizers has been moved to the tokenizers

* Fixes bug: the command `bibindex -w [index name] -m [date range]`
  previously was updating the `last_updated` field of the index

* Moves many of the index specific special cases from
  `bibindex_engine.py` to the tokenizers
  (e.g. `if index_name == "author": # add special tokens`)

* Moves some utility functions from `bibindex_engine.py` to
  `bibindex_engine_utils.py`

* Solves inveniosoftware#1662
  (BibIndex: --reindex should not zero last_updated)

* Adds tests for bibindex CLI, for bibindex utils

Signed-off-by: Federico Poli <federico.poli@cern.ch>
BibIndex: author parent tokenizer

* Adds a generic parent tokenizer, useful for the dataset records:
  implements inveniosoftware#1885

* Adds tokenizers `BibIndexAbstractParentTokenizer`,
  `BibIndexCanonicalAuthorTokenizer`,
  `BibIndexExactParentAuthorTokenizer`,
  `BibIndexParentAuthorTokenizer`,
  `BibIndexParentCanonicalAuthorTokenizer`

* Adds indexes `simpleauthor`, `canonicalauthor`, `parentauthor`,
  `parentcanonicalauthor`, `exactsimpleauthor`, `exactparentauthor`

* Adds fields `simpleauthor`, `exactsimpleauthor`

* Adds upgrade script for the database changes
  (uses upgrade_atlantis)

* Transforms `author` and `exactauthor` indexes from regular to
  virtual index

* Adds file `bibindex_engine_tokenizer_regression_tests.py`

* Edits the commands run by `--load-demo-records` in order to
  reindex the bibauthorid-dependant indexes after the `bibauthorid`
  task (for example `canonicalauthor` and `parentcanonicalauthor`)

* Adds tests for tokenizers

Signed-off-by: Federico Poli <federico.poli@cern.ch>