Implemented, pushed.  However:

The change in Sersic scale factor b using iterative solution with gamma(2n,b) gives
```
#n   b/re(old)  (new)
0.5   0.6934   0.693147
1     1.67839  1.67835
2     3.67207  3.67206
4     7.66925  7.66925
```
where we see that at lower n, the approximation is still not good to 5 decimal places.  ("Old" is the C&B 1999 approximation, and "new" the iterative solution with gamma(2n,z).)  

With the new definition of b, three of the test cases fail; in particular, the comparison with "known results", where the results do not agree to 5 decimals.  The nominal test cases use `n=[1.5,2.5]`, But even testing with only `n=4`, the tests do not pass:
```
Arrays are not almost equal to 5 decimals
Sersic profile disagrees with expected result
(mismatch 0.00410913872453%)
...
Large-shear DeVaucouleurs profile disagrees with expected result
(mismatch 0.000952598689224%)
...
Flux-rescale sersic profile disagrees with expected result
(mismatch 0.00410913872453%)
```

Should I just re-draw the test reference images with the current code?