[C++][Frame] Moved the generation of random string in a new file. Modified the default constructor of frame to produce unique frame (as long as the random strings generated are unique)

[C++][Python][parser] Added operational_frames in Model as a vector of Frame {name, parent id, placementWrtParentJoint} and exposed it in python. When parsing a fixed joint, a frame is added to the model with the name of the fixed joint