cleaned documentation, added p3m entry for allowed tolerance type

forces, simimlar to other LAMMPS KSpace styles, as explained on the

"kspace_style"_kspace_style.html doc page.

RENE: Also, can you explain more what is the difference between energy

(total) and potential (per-atom)?  Which one is more vs less accurate?

Does potential mean that the maximum of any atom's energy error is <

tolerance?  I.e. it is a max across all per-atom values?  Ditto for

field, since it is computed on a per-atom basis?

Note that not all ScaFaCoS solvers support all tolerance types.

These are the allowed values for each method:

fmm = energy and energy_rel

p2nfft = field (1d-,2d-,3d-periodic systems) or potential (0d-periodic)

p3m = field

ewald = field

direct = has no tolerance tuning :ul

tolerance type are the first values in the above list, e.g. energy

is the default tolerance type for the fmm solver.

RENE: Does the code (LAMMPS or Scafacos) generate an error

if an unsupported tolerance is specified for a method?

The {fmm_tuning} option is only relevant when using the FMM method.

It activates (value=1) or deactivates (value=0) an internal tuning

mechanism for the FMM solver.  The tuning operation runs sequentially

and can be very time-consuming.  Usually it is not needed for systems

with a homogenous charge distribution. The default for this option is

therefore {0}.

RENE: is the tuning a one-time (start-up) operation if enabled?

therefore {0}. The FMM internal tuning is performed once, when the

solver is set up.

:line

{p2nfft} = FFT-based Coulomb solver

{ewald} = Ewald summation

{direct} = direct O(N^2) summation

{p3m} = PPPM

We plan to support additional ScaFaCoS solvers from LAMMPS in the

future.  For an overview of the included solvers, refer to