Allow non-cassie models

[kkevlar]

Currently, the Cassie model is essentially hard-coded into the tool's functionality. The number of cassie bodies / joints is used throughout the program to avoid dynamic memory allocation and to perform biped-specific IK.

• Modify and validate of lines uing CASSIE_QPOS_SIZE macro to conform to using dynamic data structures.
• Remove or re-write the parts of ik.c which set only the joint positions of either the left or right leg
• Fix the node.c helper functions which use 26 as the starting index of node bodies