constraint solver seems to have issues with VBs

[AchimTuran]

Describe the bug
Somehow constraining these VBs seem to impossible with the current implementation of the constraint solver.

To Reproduce
Steps to reproduce the behavior:

1. Add those VBs to the skeleton asset

((VirtualBoneName="VB foot_root",SourceBoneName="root",TargetBoneName="root"),(VirtualBoneName="VB foot_l",SourceBoneName="VB foot_root",TargetBoneName="foot_l"),(VirtualBoneName="VB foot_r",SourceBoneName="VB foot_root",TargetBoneName="foot_r"),(VirtualBoneName="VB hand_l_to_hand_r",SourceBoneName="hand_l",TargetBoneName="hand_r"),(VirtualBoneName="VB hand_r",SourceBoneName="VB hand_l_to_hand_r",TargetBoneName="hand_r"),(VirtualBoneName="VB hand_l_to_ik_hand_gun",SourceBoneName="hand_l",TargetBoneName="ik_hand_gun"),(VirtualBoneName="VB ik_hand_r",SourceBoneName="VB hand_l_to_ik_hand_gun",TargetBoneName="hand_r"),(VirtualBoneName="VB hand_r_to_hand_r",SourceBoneName="hand_r",TargetBoneName="hand_r"),(VirtualBoneName="VB hand_l",SourceBoneName="VB hand_r_to_hand_r",TargetBoneName="hand_l"),(VirtualBoneName="VB hand_r_to_ik_hand_gun",SourceBoneName="hand_r",TargetBoneName="ik_hand_gun"),(VirtualBoneName="VB ik_hand_l",SourceBoneName="VB hand_r_to_ik_hand_gun",TargetBoneName="hand_l"))

2. Try to use these constraint settings in the IK rig

((ConstraintBone="root",ModifiedBone="ik_foot_root"),(ConstraintBone="foot_l",ModifiedBone="ik_foot_l"),(ConstraintBone="foot_r",ModifiedBone="ik_foot_r"),(ConstraintBone="root",ModifiedBone="ik_hand_root"),(ConstraintBone="hand_r",ModifiedBone="ik_hand_gun"),(ConstraintBone="hand_r",ModifiedBone="ik_hand_r"),(ConstraintBone="hand_l",ModifiedBone="ik_hand_l"),(ConstraintBone="ik_foot_l",ModifiedBone="VB foot_l"),(ConstraintBone="ik_foot_r",ModifiedBone="VB foot_r"),(ConstraintBone="ik_foot_root",ModifiedBone="VB foot_root"),(ConstraintBone="ik_hand_gun",ModifiedBone="VB hand_r_to_ik_hand_gun"),(ConstraintBone="hand_l",ModifiedBone="VB ik_hand_l"),(ConstraintBone="hand_r",ModifiedBone="VB hand_r_to_hand_r"),(ConstraintBone="hand_l",ModifiedBone="VB hand_l"),(ConstraintBone="hand_r",ModifiedBone="VB hand_l_to_ik_hand_gun"))

Expected behavior
VBs are constrained correctly.

Screenshots

Desktop (please complete the following information):

• OS: Windows
• TTToolbox Version v0.4

Additional context
Issue was found in #20.