TODO

* Tag all problems

* Make sure all problems have reasonable/descriptive "COMMENT"s

* The kind of tolerance setup and num_points thing from 1.4 should be used
  in most places, but be careful where the tolerance is already being set.
  Also, students like to plug things into calculators and round off.

* Reasonable goal is to have at least 10 relevant reasonably distinct
  problems per hour of estimated lecture.  Still not quite there yet,
  so more work needed.

* Add more distinct word problems in 2.4 and 2.6

* Add more Chapter 5 problems (only a couple in 5.1 are there now)

* Add 6.5 (PDE with Laplace) problems (only one is there now)

* More solutions, especially in the early sections

* ??? Add simple foxes and hares exercise in addition to the ones there

* In 8.1 add an exercise in 3D?

* Exercise 1 in 5.1 demands crazy precision, that should be fixed

* Make "arbitrary constants" be better checked throughout, especially in places
  where the student could come up with some more crazy expression.  See 1.4
  problem 1, 2, 4, 7 for affine versions.

* In 2.2, there are a few problems that require specific forms.  Should use the
  smarter checking procedure from the other problems (check for two linearly
  independent solutions).

* 1.4 #2: Should accept any multiple of the integrating factor and then should
  evaluate the rest of the problem with this in mind.  Also if taken a
  different antiderivative, that's the one that should be checked for in the
  final answer.  Basically allow alternatives but also check that they are
  consistent with one another.