Add support for node duplication in AMR

[grospelliergilles]

Currently when we refine a cell we only create nodes if needed. This issue is to implement a mode where we can duplicate nodes.

Duplication allows numerical schemes to have different values for nodes or faces of different levels.

For example, at the moment, if we have a cell like this at level 0

Level 0
0---------1                
|         |
|         |
|         |
3---------2

Without duplication

Level 1

0----4----1                
|    |    |
7----8----5
|    |    |
3----6----2

With duplication

Level 1

9----4----10                
|    |    |
7----8----5
|    |    |
12---6----11