Made "id" be stored in "entity_group" but not in memory in HDF5

The base "Entity" now stores and gets "id" via "entity_group" and thus all getter-ctors and getters have been modified not to pass/require id.
The only exception is "Property" which would not support "setAttribute" on "property_group" since it uses "setData" on it. For Property a memory "id" has been implemented.
Made PropertyHDF5 inherit IEntity & implement it on its own

This way all attributes are stored not on the group, which lost any purpose and has been removed, but on the dataset.
Fixed comments on BlockHDF5 & removed id in PropertyHDF5 & getProperty
Merge pull request #327 from balint42/store_id_in_groups

Made "id" be stored in "entity_group" but not in memory in HDF5