Default chunk size in single_io too large for small particle numbers

One of my GIZMO tests with only 50,000 particles crashes when writing a snapshot. Relevant part of the output:

  #010: ../../../src/H5Dchunk.c line 540 in H5D__chunk_construct(): chunk size must be <= maximum dimension size for fixed-sized dimensions
    major: Dataset
    minor: Unable to initialize object
single_io.c:writeArrayBackEnd():228: Error while creating dataspace 'Coordinates'.

The problem is solved by changing chunk_shape[0] = 1 << 16 in single_io.c to a smaller value. I suggest adding a check that resets chunk_shape[0] to N if N is smaller than 1 << 16.