schemaview.py: reorder relationships

[ialarmedalien]

I'll fix this branch up after #466 and #467 are dealt with.

Some minor edits:

Commit 1, b5733f5

• grouped the relationship-related methods according to the type of entity they deal with (e.g. put class_parents, class_ancestors, class_children, class_descendants, class_roots, and class_leaves together, and similarly for the other entity types).

Commit 2, 2989a50

• added **kwargs to the functions that call _closure(...) to allow the use of kwargs (surprise!)
• minor edits to the _closure function to simplify how it is written

[codecov]

Codecov Report

❌ Patch coverage is 86.56716% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.48%. Comparing base (08e1b1e) to head (c4ddc91).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
linkml_runtime/utils/schemaview.py	86.56%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #465      +/-   ##
==========================================
- Coverage   77.48%   77.48%   -0.01%     
==========================================
  Files          52       52              
  Lines        4464     4459       -5     
  Branches      970      968       -2     
==========================================
- Hits         3459     3455       -4     
  Misses        781      781              
+ Partials      224      223       -1     

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[ialarmedalien]

i = list.pop(0) pops the first entry off the list