Add getUnresolvedLinks method to Instance base class

[ehennestad]

Refactor and update openminds.abstract.Schema

• Fixed isReference() to support non-scalar objects
• Added getUnresolvedLinks for retrieving identifiers for all linked nodes of a metadata instance
• Added isUnresolved public method

[github-actions]

Test Results (R2021b)

548 tests  ±0   548 ✅ ±0   1m 33s ⏱️ -15s
 16 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 47eb29a. ± Comparison against base commit 9f4fb4a.

[github-actions]

Test Results (R2024b)

548 tests  ±0   548 ✅ ±0   1m 25s ⏱️ -4s
 16 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 47eb29a. ± Comparison against base commit 9f4fb4a.

[codecov]

Codecov Report

❌ Patch coverage is 31.25000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.51%. Comparing base (9f4fb4a) to head (47eb29a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
code/internal/+openminds/+abstract/Schema.m	31.25%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
- Coverage   75.73%   75.51%   -0.22%     
==========================================
  Files         339      339              
  Lines        3655     3668      +13     
==========================================
+ Hits         2768     2770       +2     
- Misses        887      898      +11     

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