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.