Use `expect` to communicate that every element should have a column
index when `pose` is called. Rewrite the panic messages in the style
recommended by the `expect` documentation.
Although the inversive distance regulator mostly acts as a general
Lorentz product regulator, calling it `InversiveDistanceRegulator`
explains how we intend to use it and demonstrates our naming convention.
This improves code organization at the cost of a little redundancy: the
default implementation of `activate` doesn't do anything, and its
implementation for `HalfCurvatureRegulator` redundantly accesses the
set point signal and checks whether the regulator is set.
Before, a `ConstraintProblem` only specified the indices of the frozen
entries. During realization, the frozen entries kept whatever values
they had in the initial guess.
This commit adds the values of the frozen entries to the `frozen` field
of `ConstraintProblem`. The frozen entries of the guess are set to the
desired values at the beginning of realization.
This commit also improves the `PartialMatrix` structure, which is used
to specify the indices and values of the frozen entries.
