Use jsonpath to collect values when downloading a file

[crisely09]

There was a problem when doing something like:

forge.download(cell_types_ontology, path=my_path, content_type='application/ld+json')

when the encodingFormat was JSON, as it was used in the headers, and the metadata of the file was return instead of the content of the file.

Solution:
Use the default headers (Accept : */*) to download the content of a file and switch to jsonpath to collect values from a given path.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.95918% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.16%. Comparing base (bf14555) to head (8a97fab).

Files with missing lines	Patch %	Lines
kgforge/core/archetypes/read_only_store.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #425      +/-   ##
==========================================
+ Coverage   74.01%   74.16%   +0.15%     
==========================================
  Files         106      106              
  Lines        7020     7065      +45     
==========================================
+ Hits         5196     5240      +44     
- Misses       1824     1825       +1     

Flag	Coverage Δ
unittests	74.16% <97.95%> (+0.15%)	⬆️

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[lecriste]

LGTM