Re: DESCRIPTOR D2Q9Descriptor
lbHelpers.h and lbHelpersD2Q9.h belong together. lbHelpersD2Q9.h contains the template specialization for the lbDynamicsHelpers class for the D2Q9DescriptorBase (and therefore for the D2Q9Descriptor). The call to lbHelpers<D2Q9Descriptor>::some_method() is forwarded to lbDynamicsHelper<D2Q9Descriptor::BaseDescriptor>::some_method. The compiler now has the generic version for that method in lbHelpers.h but an explicit template specialization in lbHelpersD2Q9.h that fits as well. As the C++ standard requires the compiler to select the explicit template specialization the code will use the lbHelpersD2Q9.h version in the end.
My suggestion for using a different equilibrium implementation would be to define a new descriptor (keep in mind that you also need to change the base descriptor) and then implement an explicit template specialization for this new descriptor (new descriptor in latticeDescriptors.h[h] and new template specialization in a new lbHelpersXY.h file (keep in mind that the new file needs to be included in lbHelpers.h)). That way you can keep your changes separate and can easily switch back to the classical equilibrium calculation.