* Add properties to the Mission class for overhead, slew
  velocity, and slew acceleration.
* Change the observation time from a binary decision variable to
  a continuous one.
* Implement the position-dependent slew constraints using a lazy
  constraint callback to keep the problem size manageable.

It seems to speed things up by about 2x.

This is normally done implicitly by CPLEX in its presolve, but
that was disabled by the lazy constraints.

Each has strengths and weaknesses. In this early stage, it is valuable
to keep around both.

Dorado uses a frame transfer CCD, so there is effectively no
readout overhead.