Improve spouted bed example

[blaisb]

Description of the problem

• The spouted bed example did not have the right specification for the particle density
• The setup of the example was suboptimal in some ways

Description of the solution

• Fix density
• Fix collisions properties and etc.

How Has This Been Tested?

• Reran it and it works just fine. It's also very fast (like 5 hours at most)

Documentation

• Updated

[voferreira]

I am not sure why, but the example is crashing on my machine.

[voferreira]

Ok, so it seems to work with bdf1, but not with bdf2