v5.13.4

ModelingToolkit v5.13.4

Diff since v5.13.3

Closed issues:

• Building ODEProblem of non-simplified, connected system: cryptic error (#890)
• default_u0 and default_p -> defaults (#898)
• structural_simplify of hierarchical system results in empty system (#906)

Merged pull requests:

• Add the MTK paper in README (#896) (@YingboMa)
• Point to ColPrac and forums within the docs (#904) (@pbouffard)
• Use defaults and variable mapping clean up (#905) (@YingboMa)