std::complex support

[steven-varga]

Complex numbers are prevalent in signal processing and more generally scientific computing. BLAS | LAPACK supports complex numbers therefore it is reasonable to have the ability to store the content of objects with complex datatypes in a straightforward way. While the HDF5 CAPI doesn't provide predefined complex datatypes ie: H5T_IEEE_C32LE is not present, the library allows creating a custom representation of such types.
Since the in memory layout of std::complex is guaranteed to be contiguous by the c++11 standard section §26.4 see this stack overflow notes the implementation is indeed straightforward and will follow pattern with the newly added half float dataypes

This section is opened to leave comments/observations for the upcoming std::complex support.