Updates to size_structure branch

[mnlevy1981]

1. ~Want x=1.05 not x = -1.05 in example~
2. ~Include units in definition of variables~
3. ~Abundance should be per size, not per mass => no need to include m' term in integral~
4. ~Use common variable names: x instead of s, alpha and beta instead of c2 and x (respectively), and cm instead of c1. Plus s0,s1,s2 -> x0,x1,x2~
5. Find analytic value of r0 (should fall out of limit as beta of the equation cm*n0 = M/N)
6. ~Implementing Newton's Method will require finding analytic form of dF/dbeta and dG/dbeta~
7. ~Matt has Newton Method solver in his box model (see co2calc module), can test size structure stuff there~

[mnlevy1981]

The "analytic value of r0" is a tough limit to find, but I think it might be as simple as r0 = r2