Get the default measure for a task type, task, task description or a learner.
Currently these are:
classif: mmce
regr: mse
cluster: db
surv: cindex
costsen: mcp
multilabel: multilabel.hamloss
getDefaultMeasure(x)
(Measure).
([character(1)` | Task | TaskDesc | Learner)
Task type, task, task description, learner name, a learner, or a type of learner (e.g. "classif").