Add bounds-checking to ObjectProperty and appropriate tests against r…

[adamkewley]

Fixes issue #3347

Brief summary of changes

• Used .at to add bounds-checking to OpenSim::ObjectProperty, so that mis-indexing into one (e.g. by calling get_property() or similar) causes a runtime exception rather than a segfault
• Added tests to testComponentInterface.cpp and testConstraints.cpp to ensure the behavior doesn't regress

Testing I've completed

• I added the test to testConstraints.cpp
• It did not segfault, because a previous fix hotfixed it (here: 4fa4493)
• I then added tests to testComponentInterface.cpp
• The SimpleProperty one did not segfault, because it was fixed in Patched segfault from reading invalid osim data (#3409) #3411
• The ObjectProperty one did segfault
• Then I added the bounds checking to Object.h (i.e. swap index accessors for .at)
• Then the test passed

Looking for feedback on...

CHANGELOG.md

• updated.

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This change is 

[nickbianco]

LGTM.

(If it fails again, the Java Moco sliding mass test can be ignored)