fix an error in momentum calculation introduced in !2898
When converting the inner product for
auto vars = {txT * txT, txT * txT * txT * txT, txT * txV, tyV * tyV, tyV * tyV * tyV * tyV};
const auto coef = std::inner_product( next( begin( params ) ), end( params ), begin( vars ), params[0] );
to
const auto coef = params[0] + params[1] * txT * txT + params[2] * txT * txT * txT * txT + params[3] * txV * tyV + params[4] * tyV * tyV * tyV * tyV;
last two terms were mistakenly converted.
It doesn't seem to have any effect on the performance.