fixes #20 make otb-app-folder setting optional

default app path is added to app dir list which is
<prefix>/lib/otb/applications

otb/OTBAlgorithmProvider.py

@@ -67,18 +67,11 @@ class OTBAlgorithmProvider(QgsProcessingProvider):
     def check_app_folder(self, v):
         if not self.isActive():
             return
-
-        if not v or not os.path.exists(v):
-            self.setActive(False)
-            raise ValueError(self.tr('Cannot activate OTB provider'))
-            return
-
-        folder = OTBUtils.otbFolder()
         
-        otb_app_dirs =  self.otb_app_dir_list(v)
-        if len(otb_app_dirs) < 1:
-            self.setActive(False)
-            raise ValueError(self.tr("Invalid path given for 'OTB application folder'. OTB provider will be disabled"))
+        folder = OTBUtils.otbFolder()
+        otb_app_dirs =  self.otb_app_dir_list(v, folder)
+        if len(otb_app_dirs) == 1:
+            utils.iface.messageBar().pushInfo("OTB", "OTB applications folder set to default:  '{}'".format(otb_app_dirs[0]))
 
         #isValid is True if there is atleast one valid otb application is given path
         isValid = False
@@ -189,7 +182,7 @@ class OTBAlgorithmProvider(QgsProcessingProvider):
                                                 ,validator=self.check_otb_folder
                                                 ))
         ProcessingConfig.addSetting(Setting(group, OTBUtils.APP_FOLDER,
-                                                self.tr("OTB application folder"),
+                                                self.tr("OTB application folder (optional)"),
                                                 OTBUtils.appFolder()
                                                 ,valuetype=Setting.MULTIPLE_FOLDERS
                                                 ,validator=self.check_app_folder
@@ -294,12 +287,15 @@ class OTBAlgorithmProvider(QgsProcessingProvider):
     def descrFile(self, d, f):
         return os.path.join(self.descrFolder(d), f)
 
-    def otb_app_dir_list(self, v):
+    def otb_app_dir_list(self, v, otb_folder):
+        default_app_dir = os.path.join(otb_folder, 'lib', 'otb', 'applications')
         folders = v.split(';')
         app_dirs = []
         for f in folders:
             if f is not None and os.path.exists(f):
                 app_dirs.append(f)
+        if not default_app_dir in app_dirs:
+            app_dirs.append(default_app_dir)
         return app_dirs
         
     def name(self):