update include conventions and refer to the new CMake build target

* the header matching the implementation (`some_class.h` for file `some_class.cpp`)

* mpi.h  (only if needed)

* LAMMPS local headers (preferably in alphabetical order)

* system and library headers (anything that is using angular brackets; C-library headers first, then C++)

* system and library headers (anything that is using angular brackets; preferably in alphabetical order)

* conditional include statements (i.e. anything bracketed with ifdefs)

### Special Cases and Exceptions

The [Include What You Use tool](https://include-what-you-use.org/)

can be used to provide supporting information about compliance with

the rules listed here.  There are some limitations and the IWYU tool

may give incorrect advice.  The tools is activated by setting the

CMake variable `CMAKE_CXX_INCLUDE_WHAT_YOU_USE` variable to the

path of the `include-what-you-use` command.  When activated, the

tool will be run after each compilation and provide suggestions for

which include files should be added or removed.

the rules listed here.  Through setting `-DENABLE_IWYU=on` when running

CMake, a custom build target is added that will enable recording

the compilation commands and then run the `iwyu_tool` using the

recorded compilation commands information when typing `make iwyu`.

## Legacy Code