Created problem_1 in hw1b/ -- just to initialize it.

Empty so far.
Wrote a function that makes problem_2d(). It works!

Although when I look at the plot I realize that I should have had my y-values explicitly subtract off the 200 mm target focal distance -- rather than have them get subtracted implicitly in the y-scaling (which is ugly and runs into the title). Eh, that's for later (i.e. never).