Refactor gramsDisplay with tests

[mortenfyhn]

A good opportunity to try out doctest

• [x] Get any test example running
• [x] Share dependency specification

[mortenfyhn]

Possible to do this?

• Convert float to string, with exactly one decimal place
• if 4 chars or less (excluding decimal separator dot)
  • return result
• try convert with no decimal place
• if 4 chars or less
  • return result
• print error

[mortenfyhn]

Done! 6146b00