[fixbug] tweak css to allow equation 4 to display correctly
[addnew] add note on top indicating what previous knowledge is assumed
[fixbug] fix capitalization of 'Cartesian'
[modify] divide the pb cgh process more explicitly in three steps
[polish] typos, punctuation and other minor fixes
[addnew] add note with reference to why tau is used instead of 2pi
[modify] link pb-cgh document to wave-propagation one; update the latter

wave-propagation.html updated to have the same indentation scheme
and suplemented with a link where hopefully additional information
can be found to complete this description.
[modify] rewrite several portions to clarify some concepts
[modify] some more rewriting
[addnew] add image to better explain the wave nodes
[modify] use time steps rather than phase steps (more intuitive)