Made DigitDisplay take a button size parameter rather than use variable, need to scale the NumEntryBox with it
Making problems into own class
scaling all the problem display stuff correctly now, they are all a little smaller than the num pad buttons.
auto_next moves to the next problem when the current one is right, but it would be nice to pause for some amount of time to show the user they got it right.
hacky way of making solved problems less likely to arise again
Scaling num entry box and digits displays to match numbers in them, though + sign is moving unaccountably.  Need to make sure numClick doesn't enter numbers bigger than base.
This avoid problem with mysteriously updating plus sign position
This change allowed placement of the num entry box and the problem indicators relative to the width of the num buttons, but I'm encountering this error when clicking on the num buttons:

Uncaught TypeError: Object [object Object] has no method 'checkAnswer' script.js:401
got around error by passing in 'pointer' to object, just passing in the function loses the identity?