fixed clang warnings about alignment change and floating point precision

This fixes two warnings that come up in the CI with Clang. The first is related to a conversion of a pointer to a std::complex value on the stack to a pointer to __m64. We can just align the res variable with the same alignment as required by __mm64, things should be safe then.

The other fix is just dropping a .f suffix for a double constant.

The remaining Clang warnings are about

Eigen/src/Core/Reverse.h:194:51: warning: implicit conversion loses integer precision: 'Eigen::Index' (aka 'long') to 'int' [-Wshorten-64-to-32]

Which is because Eigen::fix<> has int as its template parameter, even though it fulfills the function of an Eigen::Index I think. But addressing that seems to be a more complicated change, so I've left that as is for now.