[processing][needs-docs] Make Find Projection algorithm simpler to use

We no longer require the seperate CRS parameter, because the extent
parameter contains the CRS information itself. This means the algorithm
is simplified to just picking the layer and drawing the desired area
on the canvas.

python/plugins/processing/algs/qgis/FindProjection.py

@@ -40,7 +40,8 @@
                        QgsProcessingParameterFeatureSource,
                        QgsProcessingParameterExtent,
                        QgsProcessingParameterCrs,
-                       QgsProcessingParameterFeatureSink)
+                       QgsProcessingParameterFeatureSink,
+                       QgsProcessingParameterDefinition)
 from qgis.PyQt.QtCore import QVariant
 
 from processing.algs.qgis.QgisAlgorithm import QgisAlgorithm
@@ -72,9 +73,12 @@ def initAlgorithm(self, config=None):
                                                               self.tr('Input layer')))
         extent_parameter = QgsProcessingParameterExtent(self.TARGET_AREA,
                                                         self.tr('Target area for layer'))
-        #extent_parameter.skip_crs_check = True
         self.addParameter(extent_parameter)
-        self.addParameter(QgsProcessingParameterCrs(self.TARGET_AREA_CRS, 'Target area CRS'))
+
+        # deprecated
+        crs_param = QgsProcessingParameterCrs(self.TARGET_AREA_CRS, 'Target area CRS', optional=True)
+        crs_param.setFlags(crs_param.flags() | QgsProcessingParameterDefinition.FlagHidden)
+        self.addParameter(crs_param)
 
         self.addParameter(QgsProcessingParameterFeatureSink(self.OUTPUT,
                                                             self.tr('CRS candidates')))
@@ -91,7 +95,11 @@ def processAlgorithm(self, parameters, context, feedback):
             raise QgsProcessingException(self.invalidSourceError(parameters, self.INPUT))
 
         extent = self.parameterAsExtent(parameters, self.TARGET_AREA, context)
-        target_crs = self.parameterAsCrs(parameters, self.TARGET_AREA_CRS, context)
+        target_crs = self.parameterAsExtentCrs(parameters, self.TARGET_AREA, context)
+        if self.TARGET_AREA_CRS in parameters:
+            c = self.parameterAsCrs(parameters, self.TARGET_AREA_CRS, context)
+            if c.isValid():
+                target_crs = c
 
         target_geom = QgsGeometry.fromRect(extent)