FFT_2D_C2C is forced to forward transform

[Maximilian Knespel]

FFT_2D_C2C also needs an inverse equivalent. Currently it is defined as:

template< 
    typename T_Precision = float, 
    bool T_isInplace = false 
>
using FFT_2D_C2C = FFT_Definition< 
    FFT_Kind::Complex2Complex, 
    2, 
    T_Precision, 
    std::true_type, // forward hardcoded
    T_isInplace 
>;

[Alexander Grund]

Sure. It's just a matter of another alias definition. I could not come up with a nice, short name so I left it for later.