More options for the mmap keyword to load

[matthew-brett]

This is from the discussion at the developer hangout.

Yarik was wondering whether it was possible to have mmap='enforce' mode for load.

This would try and mmap the image data, and raise an error if that was not possible.

Another option Yarik wondered about was read / write memory mapping, where writing to the array changes the file on disk.

[matthew-brett]

Investigating, it is possible to use 'r+' mode to np.memmap, and this does seem to work in changing the underlying file.

[matthew-brett]

mmap PR is #268

Discussion starting from http://mail.scipy.org/pipermail/nipy-devel/2014-October/010377.html and http://mail.scipy.org/pipermail/nipy-devel/2014-November/010381.html

I see from the current code and my edits that I allow 'r+' mode in the array_from_file low-level function, but disallow it from the image class load and array proxy creation. I remember doing this explicitly, for some reason to do with the potential API problems that could result, but I don't remember what they were, and I don't seem to have recorded them anywhere.

[effigies]

It seems to me that an 'r+' option should error out when mmaping fails, rather than default to an in-memory array, since the expected behavior is that a change to the array results in a change on disk. This applies to the existing array_from_file as well as the possible change to load.

[effigies]

I think this should have been closed by #268.