Improve implementation for random groups

[brampf]

"Random-groups structures" are currently not fully implemented but skipped over instead. To fully complete the implementation of the 4.0 Standard, PrimaryHDU should be extended by some high-level code to handle those structures

[brampf]

55c33ec fixes random groups causing problems when reading the HDU

[brampf]

Not yet the perfect solution but at least the data becomes accessible by wrapping the primary HDU like shown in the corresponding test case let group = Group(file.prime)

[brampf]

Currently read only implementation. Lot's of room for improvements