Added coupling functions for ScalarGaussBonnet theory.

[FabrizioCorelli]

Proposed changes

Introduces the class representing the coupling function of Einstein-scalar-Gauss-Bonnet gravity. The class is abstract and factory creatable, so that it acts as an interface to be specialized for the different types of couplings. Two specializations are also provided: Exponential and QuarticPolynomial.

@guilara

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Further comments

[wthrowe]

Looks good.

In the future, please submit extensive review changes as separate "fixup" commits. It makes the reviewer's job easier.

[github-actions]

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