give warning when an answer's award depends on its submitted response

[dqnykamp]

This PR creates a warning if one writes an answer such as

<answer name="ans">
    <mathInput />
    <award><when>$ans</when></award>
</answer>

where the award depends on the answer's submitted response.

Creating such an answer is an easy mistake when learning DoenetML, but it leads to requiring that one submits an answer twice before it is properly validated.