update include file policy file and policy for pointers.h special case

Include files should be included in this order:

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

* mpi.h

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

* 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++)

### Special Cases and Exceptions

#### pointers.h

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`).

The `pointer.h` header file also includes (in this order) `lmptype.h`,

`mpi.h`, `cstddef`, `cstdio`, `string`, `utils.h`, and `fmt/format.h`

and through `lmptype.h` indirectly also `climits`, `cstdlib`, `cinttypes`.

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

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

and functions from the utils namespace without including the

corresponding header files.

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

for arguments. Corresponding implementation files do not need to include

those headers.

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