[Design] change storage of groups / datasets

[Vindaar]

The storage of datasets and groups within a H5FileObj and H5Group should be redone.

First of all the objects should be replaced by ref objects, because that would make all the updating, which is necessary all over the place easier. This would also allow us to get rid of a lot of annoying var arguments, where (for the user anyway) nothing interesting is done to the object, only to update some internal state of the object.

Further we currently store a ref Table (which should really be a TableRef) for each data type in each object. This should be replaced by a tree, which much better models the structure of groups and datasets within a file. This would greatly simplify iterating over datasets and groups, since the depth is evident from the tree.

[Vindaar]

The first part of this issue is addressed by #32.