Oxley: tagging a sub-region with a new tag does not work.

[LutzGross]

from esys.escript import *
from esys.oxley import Rectangle
domain=Rectangle(10,10, (-3,3), (-10,10))
X=Function(domain).getX()
m=wherePositive(X[1])
Function(domain).setTags("air", m)

throws: ValueError: getTag: invalid tag name

NOTE: when refining refined element need to inherit the tag from parent.