adding script to convert xfel cxi files to simple hdf5

CXI files are hdf5 with some formatting. The main dataset is written in two parts
and a single file only seem to contain a single frame.

This script slurps up a whole bunch of cxi files (in a dir do *.cxi), and extracts the
two image parts into a numpy array. The numpy array is then offset by the largest absolute
min value to get the whole dataset above 0 - and then converted into a uint16 which works a
lot better for compression algorithms.
Plot tweaks for the xfel data case
Shell script tweak to avoid running too many tests for too long

We don't need to do a new run for each thread number from 1 to 8...
Minor tweaks to aid debugging
Change to support 3D datasets of any type.

Before only int size types were read out. With this change any data type can be read
out from the input datafile.

Note that the input probably has to be in a 3D dataset to make sense.