Eigen3 no longer a submodule.

Treat it as any other 3rd-party dependence

.github/workflows/ba-linux.yml

@@ -51,9 +51,9 @@ jobs:
       run: |
         sudo apt-get update
         sudo apt-get install -y build-essential\
-          gfortran cmake libgsl-dev libboost-all-dev libfftw3-dev libtiff5-dev libcerf-dev \
-          qt5-default libqt5designercomponents5 qttools5-dev libqt5svg5-dev \
-          ccache
+          gfortran cmake ccache \
+          libgsl-dev libboost-all-dev libfftw3-dev libtiff5-dev libcerf-dev libeigen3-dev \
+          qt5-default libqt5designercomponents5 qttools5-dev libqt5svg5-dev
     - name: Install Python packages
       run: |
         python -m pip install --upgrade pip

.github/workflows/ba-macos.yml

@@ -49,7 +49,7 @@ jobs:
 
     - name: setup dependencies
       run: |
-        brew install cmake fftw gsl boost qt5 libtiff libcerf ccache
+        brew install cmake ccache fftw gsl boost qt5 libtiff libcerf eigen
         echo "Qt5 is installed to"
         echo $QTDIR
     - name: Install Python packages

.gitmodules

-[submodule "ThirdParty/Core/eigen3"]
-	path = ThirdParty/Core/eigen3
-	url = https://gitlab.com/libeigen/eigen.git 

eigen3 @ 0dd9643a

-Subproject commit 0dd9643ad547d3dd2e23ded1d3376d0f7bdc8ada

cmake/modules/SearchInstalledSoftware.cmake

@@ -7,15 +7,10 @@ endif()
 find_package(Threads REQUIRED)
 find_package(FFTW3 REQUIRED)
 find_package(GSL REQUIRED)
+find_package(Eigen3 3.3 REQUIRED) # no need for FindEigen3, as Eigen provides Eigen3Config.cmake
 
 find_package(Cerf REQUIRED)
 message(STATUS "Cerf found=${Cerf_FOUND} lib=${Cerf_LIBRARIES} inc=${Cerf_INCLUDE_DIR} version={Cerf_VERSION}")
-# --- Eigen3 is a git submodule; throw an error if submodule is not initialized ---
-set(EIGEN3_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/ThirdParty/Core/eigen3" CACHE INTERNAL "")
-if( NOT EXISTS "${EIGEN3_INCLUDE_DIR}/.git" )
-    message( FATAL_ERROR "Eigen3 submodule was not initialized. Please run
-                          git submodule update --init")
-endif()
 
 # --- Boost ---
 set(Boost_NO_BOOST_CMAKE ON) # prevent shortcut