Add rkIKCell creating method & Add sample of selective API calling to create constrain for rkIK

[kdaic]

Related to pull request #29, I added a function to allow interactive creation of rkIKCell.
Therefore, previously, rkIKCell was created and initialized only when rkChainRegIKCell() was executed, but with this modification, several functions are added to rk_ik.h/c and rk_ik_cell.h/c, and rkIKCell is able to be created and initialized before rkChainRegIKCell and also be editable the contents interactively.

This PR is based on the suggestion in sample #29 that the temporarily created rkIKRegister data may be replaced by rkIKCell. So, This PR also includes a new sample code ik_cell_test.c, which improves the sample code ik_attr_test.c in #29 and replaces rkIKRegister with rkIKCell.

Please check and merge if OK.

[zhidao]

I'd like to modify specifications around rkIK in response to this PR.

• rename rkIKCellInit() to rkIKCellAssign()
• rename rkChainRegIK*() to rkChainRegisterIK*()
• rename rkChainUnregIK*() to rkChainUnregisterIK*()
• rename rkChainUnregisterIKCell() to rkChainUnregisterAndDestroyIKCell()
• redefine rkIKCellInit(), which is equivalent with rkIKCellInitDefault() in the above PR
• add rkIKCellAlloc(), which is equivalent with rkIKCellCreateDefault() in the above PR
• add rkChainAddIKCell(), which is equivalent with rkChainRegIKCellDefault() in the above PR
• redefine rkChainUnregisterIKCell(), which is equivalent with rkChainUnregIKCellDefault() in the above PR

Please check if they are acceptable @kdaic Then, I'll do them and merge this branch into the main branch.

[kdaic]

@zhidao
Thanks for your suggestion and merge !

[zhidao]

I'm still unconvinced why you need to use user_defined_type.