New AssignImplicit interface: Leap side

Builds/depends on dagrt!46

• A new interface for implicit RK methods is defined in leap.rk.
• leap.rk.ButcherTableauMethodBuilder is changed to generate code for the new implicit RK interface.
• Examples and tests involving implicit RK methods are updated.
• Adds an example consisting of a fully implicit RK method.
• Documentation surrounding implicit methods is improved.
• replace_AssignImplicit is changed to support systems of equations.

This change is a version bump.

• Re-dangle requirements.txt before merge

cc: @cory