Add cmake option to switch parameters and geometry to mcbm fork.

Enabled switching to mcbm forks via:

cmake -DCBM_MCBM_PARS=ON build

And to switch back:

cmake -DCBM_MCBM_PARS=OFF build

@p.-a.loizeau, marked this as draft because I'm not sure this is something we even want. Certainly makes things easier for me to generate the online setups from these forks. But I'd much rather figure out how we can get rid of these forks.

external/CMakeLists.txt

@@ -6,6 +6,11 @@
 # it off execept for the checker.
 Option(DOWNLOAD_EXTERNALS "Download the code from the external repositories." ON)
 
+Option(CBM_MCBM_PARS "Download mCBM forks for geometry and parameters" OFF)
+if (CBM_MCBM_PARS)
+  message(WARNING "MCBM PARS ENABLED: Using mcbm forks of parameters and geometry.")
+endif()
+
 if(DOWNLOAD_EXTERNALS)
   Include(InstallFlesnet.cmake)
   Set(FLES_IPC_INCLUDE_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/flesnet/lib/fles_ipc PARENT_SCOPE)