some cleanup + additions to bulk modulus prediction example

[albalu]

I just wanted to set shuffle=False instead of True in KFold which is a better default but I ended up with:

• removing some unnecessary comments
• cleaning up (slightly)
• providing more discussion on train/test errors + plotting a train/test residual histogram
• including the plots which seem to increase the notebook size only by ~5 MB?

prerequisite PR: https://github.com/hackingmaterials/matminer/pull/222