SimpleTag part1: position optional & Representation pseudo obligatory.

Next commit shall fully implement Representation as obligatory. All tests have been rewritten to work with current state of changes.
Making "position" in SimpleTag optional again.
Made "References" in SimpleTag obligatory & added "getTime" to utils.

A "vector<DataArray>" is now required by the SimpleTag ctor. Empty DataArrays will be discarded silently, but any DataArrays not found in "Block" will cause an error.
Additionally added a "getTime" function to "util" which is now also being used by the base "Entity": this allows to safely delegate from ctors without time param to those with by passing "getTime()" for the time param.
All tests have been adapted to work with new versions.
Merge pull request #164 from balint42/fix_simpletag

Fix simpletag