v1.15.4

ControlSystems v1.15.4

Diff since v1.15.2

Merged pull requests:

• Add hysteresis nonlinearity (#1041) (@baggepinnen)
• bump DI compat (#1042) (@baggepinnen)
• fix gain margin crossing at zero freq (#1046) (@baggepinnen)
• import from right package (#1047) (@baggepinnen)
• simpler margin zero freq logic (#1048) (@baggepinnen)
• CompatHelper: bump compat for DiffEqBase to 7, (keep existing compat) (#1049) (@github-actions[bot])

Closed issues:

• Docs for margin (and marginplot): not stated if GM is in db or absolute, similarly if PM is in deg or rad (#1044)
• margin() does not compute all margins even when allMargins=true (#1045)