Merge pull request #5 from jread-usgs/master

Added info for others to contribute within the readme.md file
Merge pull request #6 from jread-usgs/master

Formatting the readme file for sub headings
Merge pull request #7 from jread-usgs/master

Fix for empty cells in input files (.wtr). Are set to NaN
fix for issue 'treat NAN as empty'