Add HDF5 loader

[adamslc]

Some complications that this loader might introduce:

• a single .h5 file frequently corresponds to several Datasets, and it would be nice if the specification in Data.toml could refer directly to a single DataSet, but preserve the hierarchal relationship between multiple DataSets.
  • More generally, I frequently work with a directory full of .h5 files, and it would be nice to preserve the relationship between all of these files as well.
• In HDF5, a dataset can have attributes, and it is important that these be accessible from DataToolkit
• The HDF5.jl package is relatively heavyweight, and so it would be good to not take on an explicit dependency of this package.

[tecosaur]

Thanks for the interest. A few things:

• You'd have to either access the whole hdf5 object as usual, or create new data sets for the subdatasets (which is actually fairly easy)
• What sort of relationship between .h5 files exists?
• If we just use h5open you'd still be able to access the attributes
• I don't know of any other appropriate packages besides HDF5, but don't worry about it being a bit big to have as a dependency, that problem's already been sorted. See https://tecosaur.github.io/DataToolkitDocs/base/stable/packages/ :)