Add CtAccumulation option to select output bitdepth (aka 16 bits)

On BM18 saving Iris 15 images (on Windows, no GPFS, 10Gbps link) max out the available saving bandwidth.

Quoting Paul Tafforeau:

A solution could apparently be to go to 25 Gb links (that would bring the network speed close to the camera speed).

Another solution, discussed already several times, is for accumulation to have the possibility in lima to go back in 16 bits instead of 32 bits by dividing the frame by the number of accumulated frames. This one would already reduce by a factor 2 the necessary network speed in accumulation without loss in data precision in the vast majority of our applications (about 80-90 % of our data). It would reduce by 2 the imprint on the data storage, both on disks and on tapes and on the network, at nearly no cost. Considering the general context, I really think that this simple development should be made as a priority for lima developments to be implemented as soon as possible (we do it anyway as one of our first processing steps in most of our tools).

Indeed, the output type of the accumulation is hardcoded to int.