Maximal direction extent

[eleftherioszisis]

@lidakanari , asked for a maximal direction extent feature in #1008 (comment)

There are two ways to achieve this:

1. Add a new feature that will return the maximum of the principal direction extents.
2. Order the existing principal_direction_extent feature from max to min. The max extent would then be equal to principal_direction_extent(direction=0)

@arnaudon , @adrien-berchet, @mgeplf

[arnaudon]

I don't have strong opinion on that, maybe sorting the output of current feature would be enough. I'm not a user of that feature, so I'm not sure of its use cases and how its output is used down the line.

[lidakanari]

Sorting the output would be sufficient for me as well. This is used for validation purposes and by default the axis=0 is outputed for the morph_stats app. So if the extents are sorted this will resolve the problem.