Add .min .mean .max .std as attributes of scalar features in the HDF5 writer and make them available to the user

[paulmueller]

Working with data on network shares can be really slow (e.g. just computing the mean of a feature). Something makes data file handling really inefficient. Are there file hashes computed or something?

[paulmueller]

HDF5-access to max, mean, min implemented in 0c947b0

[paulmueller]

Will be available in 0.47.0