Allow appending instead of overwriting an output file in relevant statistics or attribute reporting modules

[Nikos Alexandris]

We frequently report statistics for multiple maps. It would be very useful to allow appending to an existing file, instead of overwriting it, by adding a flag/parameter to relevant modules such as https://grass.osgeo.org/grass76/manuals/r.stats.html or even https://grass.osgeo.org/grass76/manuals/r.univar.html.

(The latter, though, computes the overall statistics if many maps are given as an input. Perhaps a flag could allow for a serial output of statistics for each input raster map separately.)

Here a suggested example (verified that works) for r.stats, in the context of this thread ​https://lists.osgeo.org/pipermail/grass-dev/2018-August/089377.html:

Index: raster/r.stats/main.c
===================================================================
--- raster/r.stats/main.c       (révision 72717)
+++ raster/r.stats/main.c       (copie de travail)
@@ -223,7 +223,7 @@

     name = option.output->answer;
     if (name != NULL && strcmp(name, "-") != 0) {
-       if (NULL == freopen(name, "w", stdout)) {
+       if (NULL == freopen(name, "a", stdout)) {
            G_fatal_error(_("Unable to open file <%s> for writing"), name);
        }
     }