Parallelize validate

[ssssarah]

• the traverse function being optionally overwritten on Shape was due to a change in pyshacl versions (I believe an old version had this version but a newer one no longer had it. For the version we have fixed, this function does not exist) https://github.com/BlueBrain/nexus-forge/issues/43 https://github.com/BlueBrain/nexus-forge/pull/44
• renamed usage of _transitive_load_shape_graph to _transitive_load_resource_graph
• dummy implementation of _validate_many in DemoModel
• Override pyshacl ShapesGraph implementation of def shapes to return list instead of dict_values() so that it can be picklable (for multiprocessing)
• Parallelized validate. -> cannot use threads because pyshacl uses rdflib, rdflib uses pyparsing and it is not thread safe -> shape loading occurs before creating multiple processes for validation. It would be possible to load shapes in individual processes, however:
  • "caching" of shapes becomes useless, the same shape will be queried multiple times across processes
  • Shape loading in individual processes would be best if validating resources of different types/shapes
  • The former point might not be completely true because shapes re-use each other
  • It is for sure better to load the shapes beforehand if we're dealing with a list of resources of the same type/shape. Given these reasons, I opted for loading all shapes beforehand.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.06173% with 4 lines in your changes missing coverage. Please review.

Project coverage is 74.19%. Comparing base (a1ae462) to head (19e3556).

Files	Patch %	Lines
kgforge/specializations/models/rdf/service.py	95.34%	2 Missing :warning:
...gforge/specializations/models/rdf/store_service.py	0.00%	1 Missing :warning:
kgforge/specializations/models/rdf_model.py	97.22%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #418 +/- ## ========================================== + Coverage 74.13% 74.19% +0.05% ========================================== Files 106 106 Lines 6979 6998 +19 ========================================== + Hits 5174 5192 +18 - Misses 1805 1806 +1 ``` | [Flag](https://app.codecov.io/gh/BlueBrain/nexus-forge/pull/418/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/418/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | `74.19% <95.06%> (+0.05%)` | :arrow_up: | 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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[ssssarah]

Unfortunately I ran into an issue when testing validation on 4000 resources. Long story short, pyshacl uses rdflib, rdflib uses pyparsing and it is not thread safe https://github.com/RDFLib/rdflib/issues/765 I'll look more into it