Add Batch Updates

[simon-hirsch]

Currently, the OnlineGamlss().update(...) call only takes a single new row of data. We should extend this to allow for batch updates. Caution necessary: We need to treat the discounting (forget), RSS update and the deviance updates correctly.

[fkiraly]

suggestion: could be done in two steps

1. first write this logic: if a batch is passed, internally just call update repeatedly with a single row, this brings the API to the end state
2. potentially add smarter logic for this case, under the hood, without changing the public interface

[simon-hirsch]

First step done: The Gramians allow batch updates now! MR #30