Chemistry pre-processor in CAM does not compile with NAG compiler

When we add new species to the MAM package (MAM3, MAM4 or MAM7), we need to create a new chemistry mechanism file. This mechanism file contains information about the new species: its composition, emissions and any chemical reactions.

When the model is invoked with a new chemistry mechanism file, it has to run chemistry pre-processor code to generate F90 files. These F90 files are then compiled during the model build.

As of now, the chemistry pre-processor code does't compile with the NAG compiler as the code uses non-standard "system" calls (e.g. call system(some_shell_command)). Fortran 2008 (F2008) gives a standard mechanism to make system calls through "execute_command_line" intrinsic function. F2008 standards are implemented in NAG (v5.3) but not in other compilers (such as intel/14.0). Therefore, NAG is unable to compile the chemistry pre-processor code.