Add CMake based build system for CABLE offline

[SeanBryan51]

In preparation for building CABLE with spack, @harshula, @ccarouge and I have found areas where the current build system for CABLE offline can be improved (see #188 for more details). This change adds a CMake based build system to address these issues:

• Remove temporary source directory by improving the Makefile #190
  Although this problem can be solved in the Makefile by specifying the relative paths to each source file, it requires updating all the Makefile Fortran module dependency rules which is a pain. CMake automatically handles Fortran module dependencies out of the box so doing this with CMake is much easier.
• Improve build3.sh CLI by creating switches for serial Vs mpi and debug Vs non-debug #191
  When invoking cmake, custom options, compilers and compiler flags can be configurable by setting the appropriate variables via the -D flag, for example: cmake -DCMAKE_Fortran_COMPILER="mpif90" -DCMAKE_Fortran_FLAGS="-O2 -fp-model precise" -S . -B build. These can be used for doing MPI only builds and enabling specific debug flags.
• Solve dependency problems between targets/dependencies and ensure parallel builds succeed #192
  As mentioned above, CMake automatically handles Fortran module dependencies out of the box. Parallelised builds can be done by specifying the -j flag to cmake.

The following pull requests should be merged before this pull request so that we can demonstrate binary equivalence between the executables compiled by both Makefile and CMake builds. This ensures no unwanted changes are introduced by the transition.

• Remove -O0 flag from LDFLAGS #197
• Fix LDFLAGS definition and usage #199

Edit: the above PR's have been merged - the serial and MPI binaries built by CMake (in release mode) are equivalent to the binaries built by the Makefile based build system in the main branch.

Fixes #188, #190, #191, #192

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📚 Documentation preview 📚: https://cable--200.org.readthedocs.build/en/200/

[SeanBryan51]

TODO after this pull request:

• We should list the source files in alphabetical order (the current order is so that we preserve binary equivalence with the Makefile build).
• Use find_package(MPI REQUIRED) instead of specifying mpif90 when invoking cmake for MPI case.
• Add simultaneous compilation of MPI and serial executables by creating a separate target for each executable.
• Remove MPI option from build script as the same can be achieved by specifying --target cable-mpi when invoking CMake.

[SeanBryan51]

Note, some links in the documentation are currently broken as they point to source files that were added in this pull request. To see the source files, replace main with 188-cmake-based-build-system in the url.

[SeanBryan51]

@ccarouge is it worth supporting both Makefile and CMake based build systems? Or should we deprecate the Makefile build system in this pull request?

[ccarouge]

@ccarouge is it worth supporting both Makefile and CMake based build systems? Or should we deprecate the Makefile build system in this pull request?

For the record: we only want to support one build system. So this PR should deprecate the Makefile build system.

[ccarouge]

Some minor stuff. I don't think I need to review again but I'd like the -j option for cmake to be an option for build.bash.

[harshula]

Well done! Potentially a few things we can look at later.