[xFEM] add security to avoid potential memory leek in xData
One recent commit change 'mesh' data member access of class xData. It is now a private member. Two get/set methods were added to access to this pointer. But if we read with xData a mesh from a file and choose afterword to use setMesh method to reassign the 'mesh' pointer to a new mesh instance user had to pay attention to the way he treat memory allocated done during first read from file. If he did want to avoid potential memory leak he had to call the getMesh method and with the return pointer free memory, all this before calling setMesh ... This commit choose to add a state to xData to say if the 'mesh' data member point to a memory maintained by xData or not. A new delMesh method is then in charge of freeing memory pointed by 'mesh' pointer only if this memory was allocated by xData. This function is called in setMesh to avoid memory leek. It is also called in destructor to avoid to free memory allocated by user (double free error). And it is made public to offer a simple way to free memory allocated by xData if any. Hop this make things more clear to user. For sure it is not ideal for thread safety ...