Added lamellar flow model

Added a module to the modeling section, which is the lamellar flow model
of Nye (1952). Also added verification and analytical solutions.

Defined a file directory that will be mimicked in the site navigation.

modified:   index.html
new file:   module/modeling/lamellar_flow/lamellar_flow.html
new file:   module/modeling/lamellar_flow/staggered_grid_diagram.png
new file:   module/modules_list.html
modified:   stylesheets/styles.css