update include file conventions

This purpose of this document is to provide a point of reference

for LAMMPS developers and contributors as to what include files

and definitions to put where into LAMMPS source.

Last change 2019-07-05

Last change 2020-08-31

## Table of Contents

#### pointers.h

The `pointer.h` header file also includes `cstdio` and `lmptype.h`

(and through it `stdint.h`, `intttypes.h`, cstdlib, and `climits`).

The `pointer.h` header file also includes `cstdio`, `cstddef`,

`string`, `lmptype.h`, and `utils.h` (and through those indirectly

 `stdint.h`, `intttypes.h`, cstdlib, and `climits`).

This means any header including `pointers.h` can assume that `FILE`,

`NULL`, `INT_MAX` are defined.

`NULL`, `INT_MAX` are defined, they may freely use std::string

and functions from the utils namespace without including the

corresponding header files.

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