Fix base acceleration computation without gravity

Fix base acceleration computation without gravity effect by using `copy`
to copy the object instead of a simple assignment. This is cause simple
assignment assigns the address of the object instead of the object's
value.
Add `__str__()` method to `Screw6` class
Add method to set dynamic params to zero

Add `set_to_zero()` method in `DynParams` class to set all the dynamic
parameter values to zero.
Update unit test code accordingly.
Add method to set dynamic params to zero

Add method to set dynamic parameters to zero in `FloatingRobot` class.
This method allows the links for which dynamic parameters are to be
zero.
Add floating robot test case to dynmodel unit test
Minor modifications
Add `is_symbolic` attribute

Add `is_symbolic` attribute to `FloatingRobot` and `DynModel` class.