[io] rework h5 output to use Nexus format

• save spectra / energy as nexus format (http://download.nexusformat.org/doc/html/classes/applications/NXxas.html?highlight=nxxas) /close #7 (closed)
• save xas process as nexus format (http://download.nexusformat.org/doc/html/classes/applications/NXxasproc.html#nxxasproc)
• update default path if spectra come from .h5
• add a reprocessing application to recompute process from raw data if process flow is store in the .h5 file (/close #11 (closed))