allow one to put in an explicit list of VaspHandler

atomate/vasp/firetasks/run_calc.py

@@ -71,8 +71,9 @@ class RunVaspCustodian(FiretaskBase):
     Optional params:
         job_type: (str) - choose from "normal" (default), "double_relaxation_run" (two consecutive 
             jobs), "full_opt_run" (multiple optimizations), and "neb"
-        handler_group: (str) - group of handlers to use. See handler_groups dict in the code for 
-            the groups and complete list of handlers in each group.
+        handler_group: (str or [ErrorHandler]) - group of handlers to use. See handler_groups dict in the code for
+            the groups and complete list of handlers in each group. Alternatively, you can
+            specify a list of ErrorHandler objects.
         max_force_threshold: (float) - if >0, adds MaxForceErrorHandler. Not recommended for 
             nscf runs.
         scratch_dir: (str) - if specified, uses this directory as the root scratch dir. 
@@ -176,7 +177,12 @@ def run_task(self, fw_spec):
             raise ValueError("Unsupported job type: {}".format(job_type))
 
         # construct handlers
-        handlers = handler_groups[self.get("handler_group", "default")]
+
+        handler_group = self.get("handler_group", "default")
+        if isinstance(handler_group, six.string_types):
+            handlers = handler_groups[handler_group]
+        else:
+            handlers = handler_group
 
         if self.get("max_force_threshold"):
             handlers.append(MaxForceErrorHandler(max_force_threshold=self["max_force_threshold"]))