Fix r.drain algorithm (fixes Redmine #13003)

Author: Médéric RIBREUX

python/plugins/processing/algs/grass7/description/r.drain.txt

@@ -2,9 +2,12 @@ r.drain
 Traces a flow through an elevation model on a raster map.
 Raster (r.*)
 ParameterRaster|input|Elevation|False
-ParameterString|start_coordinates|Map coordinates of starting point(s) (E,N)|(0,0)
-ParameterMultipleInput|start_points|Vector layer(s) containing starting point(s)|0|True
+ParameterRaster|direction|Name of input movement direction map associated with the cost surface|True
+ParameterString|start_coordinates|Map coordinates of starting point(s) (E,N)|0.0,0.0|False|True
+ParameterVector|start_points|Vector layer containing starting point(s)|0|True
 ParameterBoolean|-c|Copy input cell values on output|False
 ParameterBoolean|-a|Accumulate input values along the path|False
 ParameterBoolean|-n|Count cell numbers along the path|False
+ParameterBoolean|-d|The input raster map is a cost surface (direction surface must also be specified)|False
 OutputRaster|output|Least cost path
+OutputVector|drain|Drain

python/plugins/processing/algs/grass7/ext/r_drain.py

@@ -0,0 +1,40 @@
+# -*- coding: utf-8 -*-
+
+"""
+***************************************************************************
+    r_drain.py
+    ----------
+    Date                 : February 2016
+    Copyright            : (C) 2016 by Médéric Ribreux
+    Email                : medspx at medspx dot fr
+***************************************************************************
+*                                                                         *
+*   This program is free software; you can redistribute it and/or modify  *
+*   it under the terms of the GNU General Public License as published by  *
+*   the Free Software Foundation; either version 2 of the License, or     *
+*   (at your option) any later version.                                   *
+*                                                                         *
+***************************************************************************
+"""
+
+__author__ = 'Médéric Ribreux'
+__date__ = 'February 2016'
+__copyright__ = '(C) 2016, Médéric Ribreux'
+
+# This will get replaced with a git SHA1 when you do a git archive
+
+__revision__ = '$Format:%H$'
+
+import os
+
+
+def checkParameterValuesBeforeExecuting(alg):
+    """ Verify if we have the right parameters """
+    if alg.getParameterValue('start_coordinates') and alg.getParameterValue('start_points'):
+        return alg.tr("You need to set either start coordinates OR a start points vector layer !")
+
+    parameters = [alg.getParameterValue(f) for f in ['-c', '-a', '-n']]
+    paramscore = [f for f in parameters if f]
+    if len(paramscore) > 1:
+        return alg.tr("-c, -a, -n parameters are mutually exclusive !")
+    return None