Added figures (from General Exam) to the introduction:background sections.  No captions or text.
Expanded introduction outline.
Lightning detection system overview paragraphs (from forthcoming ENTLN-LIS chapter).
Global circuit text.
Added a little high level text at the very start.
Thunderstorm structure section.
The step-leader and lightning discharge process.
Corrected TeX compilation error.
Lightning types section.
Added the ionosphere section (very rough).
New bibliography entries from introduction text.
Added gec figure caption.