Adds to the equivalencies enabled in the unit registry.

These equivalencies are used if no explicit equivalencies are given, both in unit conversion and in finding equivalent units.

This is meant in particular for allowing angles to be dimensionless. Since no equivalencies are enabled by default, generally it is recommended to use set_enabled_equivalencies.

equivalencieslist of tuple

list of equivalent pairs, e.g., as returned by dimensionless_angles.