Fits-Ascii notebooks might be able to close more gracefully

[pllim]

One of the participants hit "too many open files" limit. I instructed the person to exit notebook session and restart it, which wasn't ideal, but fixed the problem. A better way is for all the notebooks to handle file I/O with with statement, which hopefully will close the file pointers gracefully (except maybe during memmap situations).

[kelle]

Seems like this is still a challenge but best solution is unclear.