Add missing "bias" method to verification interface (#330)

The BIAS method was not available with the 'get method' function, because it was missing from the name list. Therefore I added it.

Author: Joep1999

pysteps/verification/interface.py

@@ -172,6 +172,7 @@ def get_method(name, type="deterministic"):
         # categorical
         if name in [
             "acc",
+            "bias",
             "csi",
             "f1",
             "fa",