These fix a bug in the range of adjacent tasks of the same type
(and colour) which meant they merged into a single bar, regardless
of whether there was a gap or not (feature of span_where).

Also update to offer proper command-line options, the main one of
which is --expand. That splits the tasks of a thread into "expand"
lines of tasks, designed so that adjacent tasks will not be on the same
line. It is now also possible to set the size of the output graphic.

The colours have also been tweaked once more.

Reset the task debugging timers every time-step

Fixes #306.

See merge request !339

Update test kernel

Removes `doself1_density_vec_2` function. Updates `testKernel.c` to include tests for `kernel_deval_1_vec` and `kernel_deval_2_vec`.

See merge request !338

Added an option to the difffloat script to ignore a certain column if the numbers are too close to 0. Used this feature for the test27perturbed column rho_dh

Disable particle cache when vectorisation is disabled.



See merge request !337

Dopair1 vectorisation merge

Summary
-------
- Contains a vectorised version of `runner_dopair1_density`
- `runners` now have two particle caches to hold cell ci and cell cj
- The `cache` has been updated to include `max_d` which is the maximum distance into the neighbouring cell
- `cache.h` contains more functions to read two cells into cache and to read a subset of particles from each cell into the cache


See merge request !320

Added softened gravity to the M2L kernels



See merge request !336

Crucial +1 for the edge case where all particles in cell ci interact with at least one particle in cell cj.