Batch retrieve

[ssssarah]

1. Generic catching of http errors on requests now also catches the errors thrown by aiohttp

2. In the case of retrieval, there is more logic that goes into making requests (there may be a single request, just like there may be 3 of them)

   • Therefore, a different task_creator is used (can be found in BlueBrainNexus)

• The entire logic of retrieval is located inside a co-routine (async-await block), see _retrieve.
• In the case of single resource retrieval, we force a blocking call.
• In the case of multiple resources retrieval, a different task_creator is provided to BatchRequest.batch_request, one that schedules tasks that are based on the asynchronous retrieval logic.

When multiple ids are provided, if some fail but not all, for the failed ones an Action object is returned, and a Resource for the successful ones

[codecov-commenter]

Codecov Report

Attention: 64 lines in your changes are missing coverage. Please review.

Comparison is base (1a8b83f) 74.33% compared to head (5ff7927) 73.92%.

Files	Patch %	Lines
kgforge/specializations/stores/bluebrain_nexus.py	10.60%	59 Missing :warning:
kgforge/specializations/stores/nexus/service.py	33.33%	4 Missing :warning:
kgforge/core/commons/execution.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## refactor_batch #384 +/- ## ================================================== - Coverage 74.33% 73.92% -0.42% ================================================== Files 103 103 Lines 6457 6497 +40 ================================================== + Hits 4800 4803 +3 - Misses 1657 1694 +37 ``` | [Flag](https://app.codecov.io/gh/BlueBrain/nexus-forge/pull/384/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/BlueBrain/nexus-forge/pull/384/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | `73.92% <12.32%> (-0.42%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain#carryforward-flags-in-the-pull-request-comment) to find out more.

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