Set SOFA_ROOT if not set and successfully guessed (#223)

On linux (and mac) the process to guess SOFA_ROOT only print it but does not do anything with it...
So this PR set this value (while python is running) so Sofa under python can find its .so if SOFA_ROOT is not set.

Author: fredroy

bindings/Sofa/package/__init__.py

@@ -48,26 +48,29 @@
     if os.path.isdir(os.path.abspath(sofa_root_guess + '/lib' )):
         print("Guessed SOFA_ROOT: " + sofa_root_guess)
         sofa_root = sofa_root_guess
+        os.environ["SOFA_ROOT"] = sofa_root
     else:
         print("Warning: cannot guess SOFA_ROOT", 
         "Loading SOFA libraries will likely fail and/or SOFA won't find its resources.")
 
-# check if SOFAPYTHON3_ROOT has been (well) set
-sofapython3_root = os.environ.get('SOFAPYTHON3_ROOT')
-if sofapython3_root:
-    print("Using environment variable SOFAPYTHON3_ROOT: " + sofapython3_root)
-else:
-    print("Warning: environment variable SOFAPYTHON3_ROOT is empty. Trying to guess it.")
-    # try a guess from <sofapython3_root>/lib/python3/site-packages/Sofa
-    sofapython3_root_guess = os.path.abspath(os.path.dirname(os.path.realpath(__file__)) + '/../../../..') 
-    if os.path.isdir(os.path.abspath(sofapython3_root_guess + '/lib' )):
-        print("Guessed SOFAPYTHON3_ROOT: " + sofapython3_root_guess)
-        sofapython3_root = sofapython3_root_guess
+if sofa_root and sys.platform == 'win32':
+
+    # check if SOFAPYTHON3_ROOT has been (well) set, only useful for Windows
+    sofapython3_root = os.environ.get('SOFAPYTHON3_ROOT')
+    if sofapython3_root:
+        print("Using environment variable SOFAPYTHON3_ROOT: " + sofapython3_root)
     else:
-        print("Warning: cannot guess SOFAPYTHON3_ROOT", 
-        "Loading SofaPython3 modules will likely fail.")
+        print("Warning: environment variable SOFAPYTHON3_ROOT is empty. Trying to guess it.")
+        # try a guess from <sofapython3_root>/lib/python3/site-packages/Sofa
+        sofapython3_root_guess = os.path.abspath(os.path.dirname(os.path.realpath(__file__)) + '/../../../..') 
+        if os.path.isdir(os.path.abspath(sofapython3_root_guess + '/lib' )):
+            print("Guessed SOFAPYTHON3_ROOT: " + sofapython3_root_guess)
+            sofapython3_root = sofapython3_root_guess
+            os.environ["SOFAPYTHON3_ROOT"] = sofapython3_root
+        else:
+            print("Warning: cannot guess SOFAPYTHON3_ROOT", 
+            "Loading SofaPython3 modules will likely fail.")
 
-if sofa_root and sys.platform == 'win32':
     # Windows-only: starting from python 3.8, python wont read the env. variable PATH to get SOFA's dlls. 
     # os.add_dll_directory() is the new way to add paths for python to get external libraries.
     sofa_bin_path = sofa_root + "\\bin"