Add full support for prismatic joints
* type="prismatic" in URDF
* forward kinematics
* bounds
* prismatic joints axes are plotted in dotted lines, where revolute joints are plotted in solid lines
Fixes 96
Fixes 104
Ex on URDF provided by https://github.com/AhmetMericOzcan. The prismatic joint is the dotted one:
![Figure 6(1)](https://user-images.githubusercontent.com/1155586/125167013-9bce4100-e19e-11eb-9835-91365bc1d9ca.png)
When sliding the prismatic joint, we see it moving:
![Figure 5(1)](https://user-images.githubusercontent.com/1155586/125167016-9ec93180-e19e-11eb-9fcb-6081cfb8938b.png)
Additional features
* Support for arbitrary joint types in the URDF
* Fixed a bug in URDF parsing where `fixed` joints could be considered as revolute
* Support for customizable optimization algorithms. Fixes 108
Thanks to https://github.com/Klausstaler and https://github.com/AhmetMericOzcan for the support and resources!