fixed the createDataTag signature from position to positionS
made simpleTag constructor to accept position as the mandatory attribute. should fix #257
merged changes with validation and docstrings migration
moved references assignments to DataTags in TestValidate for more consistency
removed _ from position attr in simpletag constructor
added validation for refs - extent matching for simpletag
add condition on extent in simpletag validation
Merge pull request #312 from asobolev/master

Consistent create methods for tags (fixes #257)