Accept Eigen version 5 as well as 3.4.x. (#985)

# Pull Request Title

Fixes #984

---
# Change Description

Broadens the valid versions of Eigen to include 5.x.y. The search range
syntax is great for this, but only supported in Eigen 3.4.1, and I have
3.4.0, so this also needs to be supported.

Author: timspainNERSC

CMakeLists.txt

@@ -55,7 +55,11 @@ if(BUILD_TESTS)
 endif()
 
 # Eigen
-find_package(Eigen3 3.4 REQUIRED)
+find_package(Eigen3 3.4) # Try to find Eigen 3.4
+if(NOT TARGET Eigen3::Eigen)
+    # If Eigen 3.4 was not found, require any Eigen version after 3.4.1 and before 6
+    find_package(Eigen3 3.4...5 REQUIRED NO_MODULE)
+endif()
 target_link_libraries(nextsimlib PUBLIC Eigen3::Eigen)
 
 if(DEFINED PYTHON_EXECUTABLE)