fixed pep8 issues

Better pushout optimization

Note that I only went through the files and fix whatever the PEP8 script for
vim reported as incorrect, but I had not actually paid attention to the
text, so there might still be inconsistencies (like for example strange
variable names, etc.). Also only scan_opt.py is done completely, scan_op.py
still has a bunch of PEP8 inconsistency.

After the fix I only run the tests in scan_module/tests.py and they passed
so I assume I had not introduced any bug in my PEP8 fixes.

Note also a test was added for this case.

Fixed typo in opt documentation

Also completed this doc and included a minor PEP8 fix

No initial inplace

added error checking of consider_constant

changed min_informative_str to use spaces instead of tabs

this improves legibility of large graphs

Rebased new lazy ifelse

I want to order them according to the order in the sequential shell
that calls them.

As for grad and Rop, I am now taking care that if the input is a list I
return a list (regardless of the number of elements).

There was a confusion in the naming of the two branches of the ifelse.