t.rast.series: allow multiple methods/output

[sbl]

After r.series received support for multiple methods/outputs in one run, computing multiple STDS statistics with t.rast.series could get a speed up if this is handed down to ths module. Attached a diff where I also tried to incorporate changes in the test coverage, but I have no experience with the test-suit. So manual testing of the diff would be welcome. When applied, also manual should be updated...

[sbl]

Support for multiple methods / output in t.rast.series

[sbl]

In 74123:

multi method support for t.rast.series; fix #3741

[pmav99]

It seems that the previous commit introduced a SyntaxError at line 90. Please fix:

     def test_multi_stats(self):
         self.assertModule("t.rast.series", input="A",
-                          method=["average", "maximum", "minimum",
+                          method=["average", "maximum", "minimum"],

[neteler]

In 74161:

t.rast.series testsuite: missing bracket added (fixes #3741) (contributed by pmav99)