Returns “NULL” if the task is not of type “costsens”.
getTaskCosts(task, subset = NULL)(matrix | NULL).
Other task: 
getTaskClassLevels(),
getTaskData(),
getTaskDesc(),
getTaskFeatureNames(),
getTaskFormula(),
getTaskId(),
getTaskNFeats(),
getTaskSize(),
getTaskTargetNames(),
getTaskTargets(),
getTaskType(),
subsetTask()