Add markdown blocks to explain each code block

[franz825]

For didactic purposes, it would be great to add markdown blocks above each code block of both upland and fluvial components. It would contain a description of the goal of each block.

[edsaac]

Started a new branch (/markdownDocumentation) for this issue.

I've been adding explanations to blocks of code in fluvial_group.ipynb and making the plots a tad prettier.

[josiearcuri]

Duplicate of #27