This fixes commit 5d5300cd which
automatically changed all 'return' to 'pure' in our code, resulting in
broken SQL etc.

This commit refactors the codebase to add a new function called
`addDocumentsToHyperCorpus` which generalises the common pattern of
calling `insertMasterDocs` and later adding the documents via the DB
`Doc` functions.

It also "lowers" some functions to run into a plain `DBCmd`.

Fixes #258

This commit fixes the issue of the "empty Ngrams". The culprit was a bug
in the `arxiv-api` package, which was fixed by:

gargantext/crawlers/arxiv-api!2 (commits)

In a nutshell, due to the fact the relevant `Conduit` was never
completing, it was blocking the `flow` function, which was endlessly
waiting for the last part of the results to arrive.

Hence we were never calling the `flowCorpusUser`, which is the function
responsible for generating the Ngrams.

One issue is that it creates new list/docs/board/graph nodes.

This turned out to be quite heavy because of module import cycles.