Need this functionality to be able to multiply any two (not essentially same) functions. Need [`fcube_for_mul()`](https://github.com/m-a-d-n-e-s-s/madness/blob/8da56b1fc0b3d6eabe155923fb844f3430fc7d05/src/madness/mra/funcimpl.h#L1872)
Need this functionality to be able to multiply any two (not essentially same) functions. Need
fcube_for_mul()