Merge pull request #323 from balint42/master

Replaced tabs with spaces
HDF5: helper methods for links

Group has now methods for creating, deleting and renaming links.
Merge pull request #325 from balint42/fix_useless_io

Fix useless io & removed copy, swap, assignment in HDF5
Merge pull request #324 from stoewer/link-helper

HDF5: helper methods for links
Merge pull request #327 from balint42/store_id_in_groups

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