Replaced eigen Conan package with CPM.

The usage of CPMFindPackage instead of CPMAddPackage searches for
local installs of the package first and falls back to CPM if not
found.

MathLib/CMakeLists.txt

@@ -38,6 +38,7 @@ target_link_libraries(MathLib
         ${OpenMP_CXX_LIBRARIES}
         $<$<TARGET_EXISTS:LAPACK::LAPACK>:LAPACK::LAPACK>
         $<$<BOOL:${OGS_USE_LIS}>:${LIS_LIBRARIES}>
+        Eigen3::Eigen
     PRIVATE
         spdlog::spdlog
 )

scripts/cmake/ConanSetup.cmake

@@ -34,7 +34,6 @@ include(${PROJECT_SOURCE_DIR}/scripts/cmake/conan/conan.cmake)
 
 set(CONAN_REQUIRES
     boost/${ogs.minimum_version.boost}@conan/stable
-    eigen/${ogs.minimum_version.eigen}
     vtk/${ogs.tested_version.vtk}@bilke/stable
     CACHE INTERNAL ""
 )

scripts/cmake/Dependencies.cmake

@@ -95,3 +95,14 @@ if (OGS_BUILD_PROCESS_ComponentTransport
         include(scripts/cmake/iphreeqc.cmake)
     endif()
 endif()
+
+CPMFindPackage(
+    NAME Eigen3
+    GITLAB_REPOSITORY libeigen/eigen
+    GIT_TAG ${ogs.minimum_version.eigen}
+    DOWNLOAD_ONLY YES
+)
+if(Eigen3_ADDED)
+    add_library(Eigen3::Eigen INTERFACE IMPORTED)
+    target_include_directories(Eigen3::Eigen SYSTEM INTERFACE ${Eigen3_SOURCE_DIR})
+endif()

scripts/cmake/Find.cmake

@@ -82,13 +82,6 @@ else()
     include(${VTK_USE_FILE})
 endif()
 
-if(OGS_USE_CONAN)
-    set(EIGEN3_INCLUDE_DIR ${CONAN_INCLUDE_DIRS_EIGEN} CACHE INTERNAL "")
-else()
-    find_package(Eigen3 ${ogs.minimum_version.eigen} REQUIRED)
-endif()
-include_directories(SYSTEM ${EIGEN3_INCLUDE_DIR})
-
 if(OGS_USE_MFRONT)
     ## pthread, is a requirement of mfront ##
     set(CMAKE_THREAD_PREFER_PTHREAD ON)