Creating an ObjectIndex, fixes https://github.com/linkml/linkml/issues/1009

[cmungall]

Given a tree-root/container object, this will create an index,
and allow for retrieval of proxy objects that shadow domain
YAMLRoot classes. These operate in the same way, except that
object references are automatically dereferenced.

For example, given a container object following the standard
personinfo schema, an index can be created and queried:

    >>> ix = ObjectIndex(container, schemaview=schemaview)
    >>> container = ix.bless(container)
    >>> for p in container.persons:
    >>>    for r in p.has_familial_relationships():
    >>>        print(f"{p.name} {p.type} {r.related_to.name}")

Note this will work even if related_to is *not* inlined.

This means naive traversal of the object tree is not guaranteed
to be bounded, unlike with a YAMLRoot object. E.g.

    >>> person.has_familial_relationships[0].
    >>>    related_to.has_familial_relationships[0].
    >>>    related_to.has_familial_relationships[0].name

In the above, the same proxy object is reused for any
object with an identifier.

[codecov-commenter]

Codecov Report

Merging #211 (40ed5dd) into main (7a53613) will decrease coverage by 0.90%.
The diff coverage is 56.48%.

@@            Coverage Diff             @@
##             main     #211      +/-   ##
==========================================
- Coverage   63.33%   62.43%   -0.91%     
==========================================
  Files          50       52       +2     
  Lines        5501     5997     +496     
  Branches     1567     1697     +130     
==========================================
+ Hits         3484     3744     +260     
- Misses       1598     1791     +193     
- Partials      419      462      +43     

Impacted Files	Coverage Δ
linkml_runtime/linkml_model/annotations.py	90.38% <ø> (ø)
linkml_runtime/linkml_model/extensions.py	85.45% <ø> (ø)
linkml_runtime/linkml_model/mappings.py	0.00% <ø> (ø)
linkml_runtime/linkml_model/types.py	100.00% <ø> (ø)
linkml_runtime/linkml_model/validation.py	0.00% <0.00%> (ø)
linkml_runtime/utils/introspection.py	68.75% <37.50%> (-31.25%)	⬇️
linkml_runtime/utils/csvutils.py	84.61% <40.00%> (-1.50%)	⬇️
linkml_runtime/linkml_model/meta.py	52.74% <52.27%> (+<0.01%)	⬆️
linkml_runtime/linkml_model/units.py	76.00% <76.00%> (ø)
linkml_runtime/utils/pattern.py	88.57% <85.18%> (+2.85%)	⬆️
... and 12 more

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