* This method returns, if it exists, an array with only one component and as many as tuples as _field has.
* This array gives for every element on which this->_field lies, its global number, if this->_field is nodal.
* For example if _field is a nodal field : returned array will be the nodal global numbers.
* This method returns, if it exists, an array with only one component and as many as tuples as _field has.
* This array gives for every element on which this->_field lies, its global number, if this->_field is nodal.
* For example if _field is a nodal field : returned array will be the nodal global numbers.