Add interface for continuous-only solves

[keevindoherty]

Per #22 this is particularly relevant when a user does not need to do a batch solve of discrete variables. Until we have proper incremental (hybrid) solves, users should be able to specify that only fresh continuous assignments are needed (e.g. in the event that only odometry measurements are added to a factor graph). The default behavior now on main performs a heuristic check for odometry-like factors and does not perform a discrete solve when only these factors are added, but this could lead to odd behavior for certain graph inputs.