Added brief discussion on how to use presets files with CMake

[CMake shortcuts for installing many packages]:

TODO: brief discussion of the cmake command line options with presets

that Axel or Richard enabled to install sets of packages at once?

Are these just for cmake, or also ccmake and cmake-gui?

Instead of specifying all the CMake options via the command-line, CMake allows

initializing the variable cache using script files. These are regular CMake

files which can manipulate and set variables, and can also contain control flow

constructs.

LAMMPS includes several of these files to define configuration "presets",

similar to the options that exist for the Make based system. Using these files

you can enable/disable portions of the available packages in LAMMPS. If you need a

custom preset you can take one of them as a starting point and customize it to your

needs.

cmake -C ../cmake/presets/all_on.cmake \[OPTIONS\] ../cmake | enable all packages

cmake -C ../cmake/presets/all_off.cmake \[OPTIONS\] ../cmake | disable all packages

cmake -C ../cmake/presets/std.cmake \[OPTIONS\] ../cmake | enable standard packages

cmake -C ../cmake/presets/user.cmake \[OPTIONS\] ../cmake | enable user packages

cmake -C ../cmake/presets/std_nolib.cmake \[OPTIONS\] ../cmake | enable standard packages that do not require extra libraries

cmake -C ../cmake/presets/nolib.cmake \[OPTIONS\] ../cmake | disable all packages that do not require extra libraries

cmake -C ../cmake/presets/manual_selection.cmake \[OPTIONS\] ../cmake | example of how to create a manual selection of packages :tb(s=|,a=l)

NOTE: Running cmake this way manipulates the variable cache in your current

build directory. You can combine presets and options with multiple cmake runs.

[Example:]

# build LAMMPS with all "standard" packages which don't

# use libraries and enable GPU package

mkdir build

cd build

cmake -C ../cmake/presets/std_nolib.cmake -D PKG_GPU=on ../cmake :pre

:line