Add cmake files for handling submodules in 3rdparty

[tristan0x]

Use-case in some projects:

• Basalt: https://github.com/BlueBrain/basalt/compare/std-3rdparty
• NMODL: https://github.com/BlueBrain/nmodl/compare/std-3rdparty
• zee: https://github.com/BlueBrain/zee/compare/std-3rdparties (only the change in top CMakeLists.txt)

[tristan0x]

This pull-request provides:

• A fix by replacing PROJECT_{SOURCE,BINARY}DIR instead of CMAKE{SOURCE_BINARY}_DIR. It fixes the use-case when a nested CMake project included in the top project with add_subdirectory uses the hpc-coding-conventions
• a new CMake function named bbp_git_submodule in cpp/cmake/3rdparty.cmake to manage the handling of 3rd parties as git submodule. Please refer to the function documentation in 3rdparty.cmake for more details. I updated a couple of projects to take this new function into account, see my previous comment.

[tristan0x]

This pull-request provides:

• A fix by replacing PROJECT_{SOURCE,BINARY}DIR instead of CMAKE{SOURCE_BINARY}_DIR. It fixes the use-case when a nested CMake project included in the top project with add_subdirectory uses the hpc-coding-conventions
• a new CMake function named bbp_git_submodule in cpp/cmake/3rdparty.cmake to manage the handling of 3rd parties as git submodule. Please refer to the function documentation in 3rdparty.cmake for more details. I updated a couple of projects to take this new function into account, see my previous comment.

@ohm314 @matz-e @pramodk Please approve unless you have additional remarks.