Add 200-level tutorials

[chiphogg]

• 201: Results of unit computations
• 202: Common units
• 203: Magnitudes

[chiphogg]

Note to self: if we decide that we should gather all scale factors for a unit, then we'd want to make that change before we write these tutorials.

[chiphogg]

Constants would be a great 200-level tutorial --- especially using them for embedded applications with bits and bytes.

[chiphogg]

Also: abbreviated construction with unit symbols.

[chiphogg]

Also: 104 should be "math functions".