Added method to get context from model or store and another to expand the url using a context

[crisely09]

Related to this issue.

• Added methods to get a context from a store and from a model
• Added method to extend an URI using a given context

[codecov-commenter]

Codecov Report

Merging #315 (5724f24) into master (8c13274) will increase coverage by 0.03%.
The diff coverage is 74.46%.

@@            Coverage Diff             @@
##           master     #315      +/-   ##
==========================================
+ Coverage   72.63%   72.66%   +0.03%     
==========================================
  Files          88       89       +1     
  Lines        5627     5711      +84     
==========================================
+ Hits         4087     4150      +63     
- Misses       1540     1561      +21     

Flag	Coverage Δ
unittests	72.66% <74.46%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
kgforge/core/forge.py	65.38% <28.00%> (-4.67%)	⬇️
kgforge/specializations/stores/demo_store.py	38.41% <50.00%> (+0.14%)	⬆️
kgforge/core/archetypes/store.py	58.98% <60.00%> (+0.10%)	⬆️
kgforge/specializations/stores/bluebrain_nexus.py	27.01% <92.50%> (+5.27%)	⬆️
kgforge/core/commons/formatter.py	100.00% <100.00%> (ø)
kgforge/specializations/stores/nexus/service.py	56.55% <100.00%> (+0.32%)	⬆️
...sts/specializations/stores/test_bluebrain_nexus.py	93.33% <100.00%> (+0.63%)	⬆️

[crisely09]

To me, it is ready to merge.