Removed assumption that image filenames are prefixed with exactly 3 digits for
frame numbers < 1000; added code to determine minimum number of digits for
image filenames for each event directory, and write it as new property
"sigdigits" for each element of eventsSrc.list.
Removed assumption that image filenames are prefixed with exactly 3 digits for frame numbers < 1000;
added alternate versions of swap_image for optimal performance with different (zm config option) EVENT_IMAGE_DIGITS values; modified init() to use new property sigdigits.