Improved number formatting

[robbievanleeuwen]

print_results() and plotting methods now default to using engineering notation as output (thanks to quantiphy). The UnitDisplay class is also added to allow custom (and built-in) unit scaling, as well as printing the desired units next to numbers.

Closes #48.

Tasks:

• Implement string_formatter() function with tests
• Add UnitDisplay class
• Add UnitDisplay tests
• Modify print_results() to use string_formatter() and units
• Apply custom number formatting and units to plots
• Default unit
• Update docs