# singlepart to multipart algorithm
processing.runandload('qgis:singlepartstomultipart', my_PA_vl, "my_field_of_country_ISO3", "memory:my_output_layer_name") # for this step I would like to set CRS for mem layer automatically
my_output_layer_name = QgsMapLayerRegistry.instance().mapLayersByName("Multipart")[0]