Systematically compute recursive ordering of auxiliary equations

Auxiliary equations appearing in set_auxiliary_variables.m and
dynamic_set_auxiliary_series.m need to appear in recursive ordering, since
those files are used for sequential evaluation.

Previously, the recursive ordering was guaranteed by a set of ad hoc rules and
workarounds, but that would not cover certain edge cases.

With this commit, the recursive ordering is systematically computed, using a
topological sort on the directed acyclic graph whose vertices are auxiliary
equations and whose edges are dependency relationships.

Closes: #22