Support implicit nested canonical links

[scpeters]

This is a follow-up to #283 to support some models without direct child links if they contain a nested model with nested links like the following:

<model name="top">
  <model name="nested">
    <link name="link"/>
  </model>
</model>

Nested canonical links can be used implicitly, but not explicitly since the :: syntax for referencing across model boundaries will not be supported until SDFormat 1.8.

The FrameAttachedToGraph does need to store the relative name of a nested canonical link using :: syntax, so a private helper function Model::CanonicalLinkAndRelativeName is added that computes this relative name. This helper function is called by Model::CanonicalLink to avoid code duplication.

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This change is 

[codecov-commenter]

Codecov Report

Merging #341 into sdf9 will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             sdf9     #341      +/-   ##
==========================================
+ Coverage   86.22%   86.25%   +0.03%     
==========================================
  Files          59       59              
  Lines        9148     9170      +22     
==========================================
+ Hits         7888     7910      +22     
  Misses       1260     1260              

Impacted Files	Coverage Δ
src/FrameSemantics.cc	76.54% <100.00%> (+0.31%)	⬆️
src/Model.cc	83.00% <100.00%> (+0.77%)	⬆️

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[EricCousineau-TRI]

Should resolve #342

[EricCousineau-TRI]

with some minor comments

Reviewed 10 of 10 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @azeey and @scpeters)

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test/integration/model_dom.cc, line 349 at r1 (raw file):

  ASSERT_NE(nullptr, model->CanonicalLink());
  // this reports the local name, not the nested name "nested::link2"
  EXPECT_EQ("link2", model->CanonicalLink()->Name());

Is there a way to assert that this link belongs to "nested"?

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test/integration/model_dom.cc, line 364 at r1 (raw file):

  EXPECT_EQ(2u, model->ModelCount());
  EXPECT_NE(nullptr, model->ModelByIndex(0));
  EXPECT_NE(nullptr, model->ModelByIndex(1));

nit Consider making this more explicit, and more explicitly check parity with ModelByName?

EXPECT_EQ(model->ModelByName("nested"), model->ModelByIndex(0));
EXPECT_EQ(model->ModelByName("shallow"), model->ModelByIndex(1));
EXPECT_EQ(nullptr, model->ModelByIndex(2));

[scpeters]

Reviewable status: 9 of 10 files reviewed, 1 unresolved discussion (waiting on @azeey and @EricCousineau-TRI)

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test/integration/model_dom.cc, line 349 at r1 (raw file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

Is there a way to assert that this link belongs to "nested"?

added this expectation in 6c7b3eb

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test/integration/model_dom.cc, line 364 at r1 (raw file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

nit Consider making this more explicit, and more explicitly check parity with ModelByName?

EXPECT_EQ(model->ModelByName("nested"), model->ModelByIndex(0));
EXPECT_EQ(model->ModelByName("shallow"), model->ModelByIndex(1));
EXPECT_EQ(nullptr, model->ModelByIndex(2));

ok, I've added this in 6c7b3eb

[EricCousineau-TRI]

Reviewed 1 of 1 files at r2, 1 of 1 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @azeey)

[iche033]

I tested this PR with gazebosim/gz-physics#66 and gazebosim/gz-sim#258. A model with a nested link and no direct child links now successfully loads in ign-gazebo. Changes look good to me.