Crash when using a 2D grid made of 2 distributed axis

[rlacroix]

XIOS crashes when outputting a field defined on a 2D grid made of 2 distributed axis.

Further investigations show that the indexes used to extract the data received from the clients are completely wrong which causes a segmentation fault.

I am attaching a simple test case which can be used to reproduce the problem in attached mode. The code is pretty limited so it must be run on N processes with N a perfect square (for example N = 4 or N = 9).