Fix python API + small fixes

[s-mandra]

Python API is not working properly for complex arrays (see issue #31).

There is also an issue in benchmark/reference.cpp when floating point are passed to std::conj. std::conj tries to convert floating point to complex number, raising an error at lines 60 and 67. Fixed by creating a new conj function that return the same object if not a complex number (otherwise, it calls std::conj).

For the handles cTensorTranspose and zTensorTranspose removed the __Complex keyword (variables are re-casted to either FloatComplex and DoubleComplex anyway).

[s-mandra]

Tested with g++-12, clang-9 and clang-13 on linux.

[ultimatile]

Regarding the fix for issue #21, how about using an explicit type conversion like hptt::DoubleComplex(__real__ alpha, __imag__ alpha) instead of (hptt::DoubleComplex) alpha?
Is it better to remove the __Complex keyword?