Missing leading slash in input file leads to h5-errors

Wasted quite a decent amount of @sanathkeshav and my time because I forgot to add the leading '/' at the beginning of the microstructure's dataset name in the input file. This led to very ugly and unclear h5-errors when running FANS. Would be nice to make this a bit more robust by ensuring that this is added automatically in case dumb users (like me) forget to put it...