[nnenum] Better detection that actually fails when nnenum is not found.

Also provide more general python script that detects the existance of a python
module. The script takes two arguments: module name as first argument, and the
output string (typically, a version) that CAISAR uses for detection as second argument.

bin/dune

@@ -2,5 +2,6 @@
  (package caisar)
  (section bin)
  (files
+  (findmodule.py as findmodule.py)
   (nnenum.sh as nnenum.sh))
 )

bin/findmodule.py

+#!/usr/bin/env python3
+###########################################################################
+#                                                                         #
+#  This file is part of CAISAR.                                           #
+#                                                                         #
+#  Copyright (C) 2022                                                     #
+#    CEA (Commissariat à l'énergie atomique et aux énergies               #
+#         alternatives)                                                   #
+#                                                                         #
+#  You can redistribute it and/or modify it under the terms of the GNU    #
+#  Lesser General Public License as published by the Free Software        #
+#  Foundation, version 2.1.                                               #
+#                                                                         #
+#  It is distributed in the hope that it will be useful,                  #
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of         #
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the           #
+#  GNU Lesser General Public License for more details.                    #
+#                                                                         #
+#  See the GNU Lesser General Public License version 2.1                  #
+#  for more details (enclosed in the file licenses/LGPLv2.1).             #
+#                                                                         #
+###########################################################################
+
+import sys
+import importlib
+
+module_name = str(sys.argv[1])
+output = str(sys.argv[2])
+
+spec = importlib.util.find_spec(module_name)
+
+if spec is not None:
+  print(output)
+  exit(0)
+else:
+  exit(1)

bin/nnenum.sh

@@ -22,7 +22,7 @@
 ###########################################################################
 
 if [ "$1" = "--version" ]; then
-    echo dummy
+    findmodule.py "nnenum" "dummy"
 else
     OMP_NUM_THREADS=1 OPENBLAS_NUM_THREADS=1 python3 -m nnenum.nnenum "$@"
 fi