add a paragraph to the output howto explaining how ambiguity is resolved

There are four basic kinds of LAMMPS output:

* :doc:`Thermodynamic output <thermo_style>`, which is a list

  of quantities printed every few timesteps to the screen and logfile.

* :doc:`Thermodynamic output <thermo_style>`, which is a list of

  quantities printed every few timesteps to the screen and logfile.

* :doc:`Dump files <dump>`, which contain snapshots of atoms and various

  per-atom values and are written at a specified frequency.

* Certain fixes can output user-specified quantities to files: :doc:`fix ave/time <fix_ave_time>` for time averaging, :doc:`fix ave/chunk <fix_ave_chunk>` for spatial or other averaging, and :doc:`fix print <fix_print>` for single-line output of

  :doc:`variables <variable>`.  Fix print can also output to the

  screen.

* Certain fixes can output user-specified quantities to files:

  :doc:`fix ave/time <fix_ave_time>` for time averaging,

  :doc:`fix ave/chunk <fix_ave_chunk>` for spatial or other averaging, and

  :doc:`fix print <fix_print>` for single-line output of

  :doc:`variables <variable>`.  Fix print can also output to the screen.

* :doc:`Restart files <restart>`.

A simulation prints one set of thermodynamic output and (optionally)

scalar values as input can typically also process elements of a vector

or array.

.. _disambiguation:

Disambiguation

--------------

Some computes and fixes produce multiple kinds of data, e.g. a global

scalar and a global vector, or a global scalar and a per-atom vector

(i.e. one value per atom).  Usually the context in which those are used

determines which datum is accessed.  Example: if a compute provides both

a global scalar and a per-atom vector, the former will be used when

using ``c_ID`` in an equal-style variable, while the latter will be

referenced as ``c_ID`` in an atom-style variable.  While atom-style

variables are otherwise also able to access global scalars, access to

the global scalar of the compute is not directly possible in this case.

This is resolved through first defining an equal-style variable that

accesses the scalar property and then access that equal-style variable

in the atom-style variable, which then may then also access the per-atom

property.

.. _thermo:

Thermodynamic output