Added support for dual sidebands.

Changed amisa_control to send datasets with n_chans*n_sidebands entries.
Updated config files for different sideband configurations.
Changed corr_grab_h5 to send datasets with n_chans*n_sidebands entries
Changed file_writer to write n_chans*n_sidebands channels and set n_chans,
center_freq and bandwidth attributes appropriately.