[REQUEST] Add get_chain_as* methods to DQ_WholeBody

[mmmarinho]

Hello, @bvadorno,

Owing to some peculiarities of Python (mixed with some CPP peculiarities), it would be convenient to have the following methods in DQ_WholeBody

DQ_SerialManipulator get_chain_as_serial_manipulator(int index)
DQ_HolonomicBase get_chain_as_holonomic_base(int index)
...

for all concrete subclasses of DQ_Kinematics.
The methods have to check if the conversion is possible and generate an error if the conversion is impossible.

E.g.:

• https://github.com/dqrobotics/cpp/blob/af8871415df93beb450917f69c50758b98c5e002/src/robot_modeling/DQ_WholeBody.cpp#L50
• https://github.com/dqrobotics/cpp/blob/af8871415df93beb450917f69c50758b98c5e002/src/robot_modeling/DQ_WholeBody.cpp#L61

The main reason for this requirement is that the Python bindings are not managing the memory well (it's freeing pointers it shouldn't be freeing), so it is simpler for us to return a copy instead of a reference, and only copies of concrete classes are possible.