Overload ScalarParameter methods for reaction model compatibility

[cbix]

When using ScalarParameter in a reaction model, the code generated from the configuration wouldn't compile because it generates code with invalid function headers, e.g.

inline void reserve(unsigned int nReactions, unsigned int nComp, unsigned int nBoundStates)
{
  _Kh20.reserve(nReactions, nComp, nBoundStates);
  // [...]
}

This PR fixes the situation by overloading the existing headers with ones compatible with code generated from the reaction model codegen template. It is required for #413.

[github-actions]

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[jbreue16]

thanks for the contribution! Can be merged imo, after two small documentation changes