use precompiler directives to merge makefiles

[edoyango]

Currently use a bash script to call seperate makescripts to compile GPU, vs CPU serial, vs CPU parallel codes. After CAF CPU parallel code, could merge CPU serial and CPU parallel into one code base, and use conditional compilation with preprocessor directives.

After than, need to think of a scheme to compile GPU code within the same makefile too.

Code bases could potentially be merged using preprocessor directives. e.g., there could be a single main.f90 file, but depending on the preprocess directives, make could compile/link the CPU or GPU version of time_integration.f90 etc.

code base probably couldn't be completely merged because of the significantly different calls made in the core computation subroutines in the GPU and CPU versions.

[edoyango]

src_CAF can work in both serial and parallel with a single makefile. Still need to integrate src_GPU.

[edoyango]

Adding another goal:
Make main.cuf/f90, input_m.cuf/f90, output_m.cuf/f90, summary_m.cuf/f90` use the same files for both caf and gpu versions