Minor changes to the test func. for all_world2pix

1. Added more parameters to the test function;
2. Changed default 'fame' to None => load 'data/sip.fits'. If fame is a
file name - that file will be opened.
3. Improved aesthetics to meed Astropy standards.
Added 2 parameters to the test function

Added `maxiter` and `detect_divergence` parameters to the
`test_all_world2pix` function.
Minor improv. to prints; extra parameters

Added `detect_divergence` and `maxiter` to the list of parameters of
the test function of `all_world2pix`.
Improvement in divergence detection; cosmetics

1. Previous implementation of my algorithm had the following problem:
detection of the divergence was happening *after* the divergent
correction has been applied to the current solution. However, one of
the main benefits of having divergence detection is to be able to get
the "best" solution for the diverging points. This "best" solution must
be the last converging solution *just before* the solution started
diverging. So, previous implementation was one iteration too late to
retain the "best" solution for the diverging points.
2. Corrected use of white spaces and backlashes per suggestions by
@embray and @mdboom.
3. Several small performance improvements.
4. Corrected documentation formatting and minor typos. Updated examples
with the output corresponding to the changes made in this commit.