Move get_var_value_as_double up hierarchy, as it was inadvertently duplicated as a non-override

[PhilMiller]

This method exists in both Bmi_Module_Formulation (from which it's overridden by the language-specific implementations) and Bmi_Multi_Formulation, with the exact same interface and apparent intended use.

Changes

• Declare get_var_value_as_double at the common base class, not separately in different child classes

Testing

• Ordinary CI

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[program--]

I don't think we hit this case, but we don't have any diamond inheritance schemes, correct? (i.e. needing virtual inheritance on either Bmi_Formulation or Bmi_Module_Formulation) -- Particularly w.r.t. the BMI base class. (This is probably out of scope for this PR, but still partly relevant, so I figured I'd ask in case we noticed this) -- Other than that, this looks good to me.

[PhilMiller]

There is some multiple inheritance in the current code (which I'm not thrilled about), but I don't think any of it hits diamond inheritance; it's all just interface mix-in sorts of things.

Thanks for the quick review