mapping loading with mapping type specified or different method name

[ssssarah]

Adresses #332

I guess the real issue is that a mapping may be a url, txt or a file path, and if it was neither a url (= couldn't request it) or a filepath (= couldn't load the file), then it was assumed to be a txt mapping. But it could have been intended to be a url but malformed, or a filepath but malformed.
Maybe some validation that whatever string is passed is meant to be a text mapping should be done. I've put a hack of a check that it should start with a {, but I'm not even sure this is true of all text mappings.

Update: in checking the validity of the mapping str provided, it brought out that many current mappings in tests are not valid paths, and the failure to load them was silent and did not make the tests fail

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (ee54140) 73.45% compared to head (7d677cc) 73.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
+ Coverage   73.45%   73.87%   +0.41%     
==========================================
  Files          91       92       +1     
  Lines        5831     5913      +82     
==========================================
+ Hits         4283     4368      +85     
+ Misses       1548     1545       -3     

Flag	Coverage Δ
unittests	73.87% <91.42%> (+0.41%)	⬆️

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

Files	Coverage Δ
tests/conftest.py	98.98% <100.00%> (ø)
tests/specializations/mappings/test_mappings.py	100.00% <100.00%> (ø)
tests/specializations/models/test_demo_model.py	100.00% <100.00%> (ø)
tests/specializations/models/test_rdf_model.py	100.00% <100.00%> (ø)
...sts/specializations/stores/test_bluebrain_nexus.py	93.90% <100.00%> (+0.03%)	⬆️
utils.py	33.33% <100.00%> (+33.33%)	⬆️
kgforge/specializations/mappings/dictionaries.py	90.00% <85.71%> (-2.31%)	⬇️
kgforge/core/archetypes/store.py	59.36% <75.00%> (+0.37%)	⬆️
kgforge/specializations/stores/demo_store.py	39.41% <75.00%> (+0.99%)	⬆️
kgforge/core/archetypes/mapping.py	81.69% <90.24%> (+11.69%)	⬆️

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[crisely09]

Looks good