Don't force-set Python cache variables

Allow the user to override Python paths
from the command line.

cmake_modules/IMPFindPython.cmake

@@ -4,18 +4,18 @@ function(imp_find_python)
 
   if(Python3_Interpreter_FOUND AND Python3_Development_FOUND)
     # Use Python 3 tools
-    set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE} CACHE INTERNAL "" FORCE)
+    set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE} CACHE STRING "")
     set(PYTHON_TEST_EXECUTABLE ${Python3_EXECUTABLE} CACHE STRING "")
-    set(PYTHON_LIBRARIES ${Python3_LIBRARIES} CACHE INTERNAL "" FORCE)
-    set(PYTHON_INCLUDE_DIRS ${Python3_INCLUDE_DIRS} CACHE INTERNAL "" FORCE)
-    set(PYTHON_LIBRARY_DIRS ${Python3_LIBRARY_DIRS} CACHE INTERNAL "" FORCE)
-    set(PYTHON_NUMPY_FOUND ${Python3_NumPy_FOUND} CACHE INTERNAL "" FORCE)
+    set(PYTHON_LIBRARIES ${Python3_LIBRARIES} CACHE STRING "")
+    set(PYTHON_INCLUDE_DIRS ${Python3_INCLUDE_DIRS} CACHE STRING "")
+    set(PYTHON_LIBRARY_DIRS ${Python3_LIBRARY_DIRS} CACHE STRING "")
+    set(PYTHON_NUMPY_FOUND ${Python3_NumPy_FOUND} CACHE STRING "")
     set(PYTHON_NUMPY_INCLUDE_DIR ${Python3_NumPy_INCLUDE_DIRS}
-        CACHE INTERNAL "" FORCE)
-    set(PYTHON_VERSION ${Python3_VERSION} CACHE INTERNAL "" FORCE)
-    set(PYTHON_VERSION_MAJOR ${Python3_VERSION_MAJOR} CACHE INTERNAL "" FORCE)
-    set(PYTHON_VERSION_MINOR ${Python3_VERSION_MINOR} CACHE INTERNAL "" FORCE)
-    set(PYTHON_VERSION_PATCH ${Python3_VERSION_PATCH} CACHE INTERNAL "" FORCE)
+        CACHE STRING "")
+    set(PYTHON_VERSION ${Python3_VERSION} CACHE STRING "")
+    set(PYTHON_VERSION_MAJOR ${Python3_VERSION_MAJOR} CACHE STRING "")
+    set(PYTHON_VERSION_MINOR ${Python3_VERSION_MINOR} CACHE STRING "")
+    set(PYTHON_VERSION_PATCH ${Python3_VERSION_PATCH} CACHE STRING "")
   else()
     message(FATAL_ERROR "Could not find a Python interpreter and matching headers/libraries. Python is required to build.")
   endif()