derived unit coercion

[mkhan45]

LaTeX output should be able to show derived units. The print expression should then look like

expr = ? (Unit1, Unit2)

[mkhan45]

For now I'm just adding a unit hint so just

expr = ? Unit

[mkhan45]

Unit hints in d6eef53

[mkhan45]

What I mean by derived unit coercion is that the program should figure out how to construct the output reasonably by using the given derived units.

Example:
9.8 m/s^2 * 40 kg * 4 seconds and Newton as a preferred unit should output with unit Newton seconds. This example would be pretty easy to implement by just trying to divide the unit by Newtons.

4 seconds / (9.8 m/s^2 * 40 kg) with Newton as a preferred unit should output with unit seconds/Newtons.

I don't think this problem is perfectly solvable but it's still worth implementing with hard Unit hints as an override