Add Gaussian plus constant constraint gamma for ScalarTensor

[guilara]

Proposed changes

I want add the Gaussian plus constant for the constraint gammas of the CurvedScalarWave system.

(Is there a way the to avoid so much duplicate code? Some member functions of the gh classes are private so I think I cannot wrap them.)

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Further comments

[knelli2]

@guilara Do you want to make this into a full PR?