Move list splitter utility into combine_experiments.py

[ndevenish]

This was causing an dependency on xfel, by transitive imports:

• combine_experiments.py
• dxtbx.command_line.image_average (for splitit, but then pulls in...)
• dxtbx.format.cbf_writer, which pulled in
• xfel.cftbx.......

This version came from stackoverflow, and was tested for all input lengths i=0...1000 and for every input, every sublength from n=1..i, to be identical, with:

def test_splits():
    for i in range(1000):
        print(i)
        for n in range(1,i):
            data = list(range(i))
            a = splitit(data, n)
            b = list(_split_equal_parts_of_length(data, n))

            assert a == b