Atomic refactor: all 'lam' variables become 'wavelength' variables.

I ran 2b and 2c and they still "work" the same.
Refactored: now inverse_fc... is gone and we just return 1/inverse_fc.

Slight cleaning up otherwise.
I moved fc_per_wavelength out of problem_2c, and renamed things.

In particular, `r1r2` -> `lens_curvatures`.