Add __str__() and __repr__() for `Screw` class
Add methods in `FloatingRobot` class

Add methods in `FloatingRobot` class for the purpose dynamic model
computation.
Add first forward recursion for inverse dynmodel

Add first forward recursion for inverse dynamic model computation.
Add helper functions to perform the various steps of inverse dynamic
model computation. Note that these helper functions maybe used by the
direct dynamic model as well.
Add `DynModel` data structure class to hold the various components of
dynamic model computation. This class is to be further extended.
Add backward and 2nd forward recursion for IDyM

Add backward recursion for inverse dynamic model computation.
Add second forward recursion for inverse dynamic model computation.
Add more helper functions to compute the inverse dynamic model.
Expand `DynModel` to include more attributes.
Refactor `__str__()` in `DynModel` class

Refacto `__str__()` in `DynModel` to return all the attributes of the
class. Since attributes can be dynamically added, the current version
uses `dir()` to return all the attributes and then convert them to
string.
Update UI for Visualisation window