Tensor modifier may map a temporary #287

include/MGIS/Function/TFEL/TensorModifier.ixx:32-35, with EvaluatorModifierBase.ixx:50, 58, 66, 76.

Finding. apply maps a TFEL tensor view onto values.data(), where values is bound to the prvalue returned by the wrapped evaluator. When that evaluator returns by value, the view points at a destroyed temporary. The requirement that gates the modifier only asks for a data() member.

Effect. Re-interpreting the output of a unary operation or a transform as a TFEL tensor reads dead stack. Reproduced under the address sanitiser; at higher optimisation levels the values happen to be right.

Fix. Map when the result is a reference or a lightweight view, copy otherwise.