Make a robust collection of trajectory types

[elidupree]

Currently, the TimeSteward library only provides QuadraticTrajectory. Ideally, it should provide a variety of trajectory types, customizable by:

• How many dimensions they are
• What degree of polynomial they are (linear versus quadratic, etc.)
• Whether they allow rounding error
• The base numeric type (Possibly anything that implements num::Integer?)

QuadraticTrajectory has one specific choice for each of these (2, 2, yes, i64). For rustweb-test-game, I had to implement different sets of choices (1-2, 1, no, i64). Client code shouldn't have to implement such basic stuff. And they should be full-featured (e.g. implementing arithmetic operators, letting you access the dimensions of a multi-dimensional trajectory as single-dimensional trajectories, etc.)