Add ExpressionBasedFunction

[nickbianco]

Fixes issue #3812

Brief summary of changes

Adds ExpressionBasedFunction for creating OpenSim Functions that leverage Lepton's expression parsing.

Testing I've completed

Add tests to testFunctions.cpp.

Looking for feedback on...

CHANGELOG.md (choose one)

• updated.

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This change is 

[aymanhab]

Few minor questions but looks great

Reviewed 9 of 9 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @AllisonJohn and @nickbianco)

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OpenSim/Common/ExpressionBasedFunction.h line 59 at r1 (raw file):

    OpenSim_DECLARE_PROPERTY(expression, std::string, 
            "The mathematical expression defining this Function.");
    OpenSim_DECLARE_LIST_PROPERTY(variables, std::string, 

Space separated? Case sensitive? Quoted or not? allowed/disallowed names

On a related note, if the definition is invalid, when does it blow up?

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OpenSim/Common/ExpressionBasedFunction.cpp line 116 at r1 (raw file):

    Lepton::ExpressionProgram m_valueProgram;
    std::vector<Lepton::ExpressionProgram> m_derivativePrograms;
};

The prefix SimTK outside the namespace is a bit uncommon is that intentional? How would it show in documentation/doxygen?

Also is it in the style guide to use the m_ prefix?

[nickbianco]

Thanks @aymanhab! Ready for review again.

Reviewable status: 7 of 9 files reviewed, 2 unresolved discussions (waiting on @AllisonJohn and @aymanhab)

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OpenSim/Common/ExpressionBasedFunction.h line 59 at r1 (raw file):

Previously, aymanhab (Ayman Habib) wrote…

Space separated? Case sensitive? Quoted or not? allowed/disallowed names

On a related note, if the definition is invalid, when does it blow up?

List properties in XML will be space separated. I added some documentation about allowed/non-allowed variable names.

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OpenSim/Common/ExpressionBasedFunction.cpp line 116 at r1 (raw file):

Previously, aymanhab (Ayman Habib) wrote…

The prefix SimTK outside the namespace is a bit uncommon is that intentional? How would it show in documentation/doxygen?

Also is it in the style guide to use the m_ prefix?

This class is internal and will be hidden from Doxgyen. See MultivariatePolynomialFunction for a similar implementation.

There's nothing in DEVELOPING.md or CONTRIBUTING.md about variable name prefixes.

[aymanhab]

Reviewed 3 of 3 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @AllisonJohn)

[nickbianco]

Thanks @aymanhab!