Fix 'weightEvents' after changes to 'fitResult'

'weightEvents' did no longer work after the matrix class in 'fitResult'
was changed to 'complexMatrix', and thus 'std::complex<>' were stored in
the fit result ROOT file. Force loading of the 'std::complex<>'
dictionary not only in the case of ROOT amplitudes.

(cherry picked from commit 425d9a9499e0f4d7fe15873cd818c9576935f3b2)