Issue with the complex numbers

There is no standard representation of the complex numbers std::complex<T> in hdf5.

The classes therefore represent them as arrays of T with one more dimension, and adding a tag _complex_ to the array for identification when rereading.

• This is compatible with the ALPS 2.0 storage.
• This is not compatible with the regular h5py storage for complex. The conversion is made by HDFArchive class automatically.