get method treatment of double arrays in mppnp se file columns

[fherwig]

on astrohub.csa there is a notebook mppnp-double-column-issue.ipynb in /home/user/repos/3d1d-advection-paper/notebooks/Post-processing demonstrating the problem. mppnp se output files can have instead of one array a list of multiple arrays. This may be the case, for example when an automatic restart was made, or when the previous H5_out dir was not removed. In any case, maybe something in the get method has changed so that this now appears as a problem. But the get method will return a list of 2 arrays, and that causes functions and methods not to work.

The behaviour should be that the get method is silently returning the last array of there are more than one. It may return a warning in that case, behind a verbose flag.

See notebook for details.