map the hdf5 files into the data API

[mmaelicke]

The hdf5 files are interfaced by xarray.

The two interface functions should do the following:

• DataSource.read should return a xarray.Dataset
• DataSource.filter: here we need to check out, how the RUINSapp uses filters. It's either filtered by dimension (timestamp) or filtered by variable. The first will preserve data-type while the latter might return as pandas.Series or pandas.DataFrame.