Release 0.3.1

Added:
 - Created a user manual
 - Create a design document (a document detailing the program's design for programmer use)
Changed:
 - Changed the name of the project from "Unit Converter" to "7Units"
   - Changed the main package from ~unitConverter~ to ~sevenUnits~
 - Tweaked the README, removing content that belongs in the new user manual
 - Renamed ~SI~ to ~Metric~
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