Write helper function to unlock the number of opened files ....

[kif]

With the size of the experiment increasing, it is common to have 1000000 images in 1000 files ...
thus one encounters a problem with the number of opened files which is limited, at least under unix.

One (half) solution is to set ulimit unlimited as done here :

https://github.com/silx-kit/silx/blob/755fa6d3cbe35e1078f65557f6e9e8352b826da5/src/silx/app/view/main.py#L106-L118

It would be nice to have one helper function that would be available in fabio for doing this.