Unset python variables doesn't allow for custom python install location

The cmake python file unsets the python libraries every time. I think the intention is to only do this if the python version changes from 2 to 3 in file nektar/cmake/ThirdPartyPython.cmake

commenting out the unset function allows custom python install locations to be set without overwriting every time.

finding a smarter way to do this or removing it would be helpful, I know that VTK has something that sets the python version maybe there is something in that project that might point the way.

--- a/cmake/ThirdPartyPython.cmake
+++ b/cmake/ThirdPartyPython.cmake
@@ -12,10 +12,10 @@ IF (NEKTAR_BUILD_PYTHON)
 
     # Unset any existing python executable/library settings so that
     # we can rediscover correct python version if v2/3 settings changed
-    unset(PYTHON_EXECUTABLE CACHE)
-    unset(PYTHON_INCLUDE_DIR CACHE)
-    unset(PYTHON_LIBRARY CACHE)
-    unset(PYTHON_LIBRARY_DEBUG CACHE)
+    #unset(PYTHON_EXECUTABLE CACHE)
+    #unset(PYTHON_INCLUDE_DIR CACHE)
+    #unset(PYTHON_LIBRARY CACHE)
+    #unset(PYTHON_LIBRARY_DEBUG CACHE)