Add more warnings about score ranges (and reject_score)

[gkreitz]

In these new checks, we compute max scores for test data groups based on the configuration of their children, and add several new warnings:

• If the range can be narrowed (e.g,. 10 points per test case, 7 test cases, but your range says 0 100)
• If no range was specified, but we can compute one
• If no range is specified on the root node (where you basically always want to specify something)
• If the root node has a negative minimum (which is almost always a mistake)

I decided not to add warnings if the computed range is larger than the declared range (e.g., 11 test cases worth 10 each, but the range says 0 100). This should be caught by out-of-bounds checks when running submissions. Happy to take opinions on if we want to check it statically here too.

Note that these checks all disappear when a custom grader is used (for obvious reasons). And if custom scoring is used, we can't make any assumptions regarding what each test case is worth, so the checks are less likely to catch stuff.

I also added an explicit warning for using reject_score. It's been removed in the new format. I cannot think of a good use case, and it's always the default 0 in all of the problems ever installed on Kattis.

Fixes #439