add lemma relating task cost and task rbf
Currently, we have lemmas stating that task_rbf
at \vareosilon
is greater than equal to task cost, and task_rbf
is monotone. I combined these two lemmas to state that tsk_rbf
at any point greater than 0
is greater than task cost. I find that this fact is regularly required in proofs and therefore, would be good to have in the facts folder.