Added variable mu per limb

[ansue1234]

Edit: Squashed Commits into 1

Just want to check whether my implementation is correct and whether it works for the unreleased examples.
Changes:

• added mu attribute to elasticRod class
  changed the function definition of addLimb for soft robot class to include mu, but mu is an optional parameter with default of 0
• tweaked contactForce class, between two limbs, use the max mu of the two limbs in contact (maybe average might be better? or so some more sophisticated logic in future)
  • In contactForce class, the option to use friction or not is now a boolean flag to be passed in
• tweaked floorContactForceClass to use mu of limb when calculating force and jacobian (a bit unsure of the correctness of my implementation on this part)
  included one shell file
  • I find the shell files useful because I can choose which Cpp file in my workspace "custom_bots" to be robotDescription.cpp, change directory to build to build and link that file, change back to the root directory, and run ./dismech shell script in 1 command. Make life a little bit easier before we can create robots via XML or urdf.

[QuantuMope]

Looks good for the most part, but I would like the friction coefficient for the floor to be readded.

It's very possible for the limbs and floor to have different friction coefficients. We don't want to remove this functionality.