pybootchartgui: _parse_proc_ps_log rewrite with iterator

Iterators use much less memory, so larger bootcharts
may be processed without triggering OOM killer and
massive swapping.

On a (big) 11MB tarball this will have a performance penalty
of about ~10% but consuming half the memory.

Before:
23.50user 1.20system 0:24.97elapsed 98%CPU (0avgtext+0avgdata 770048maxresident)k

After:
26.78user 0.44system 0:27.24elapsed 99%CPU (0avgtext+0avgdata 321192maxresident)k