Refactor DEM to remove function pointers

[blaisb]

Description of the problem

• The DEM solver makes some use of function pointers something for which there is really no use for and which we do not use in the code.

Description of the solution

• Remove the function pointer and replace them with straightforward if statements.

Comments

• This is part of a long term effort to streamline even more the DEM and the CFD-DEM solver

[blaisb]

@voferreira this is ready for a second review. I refactored the way the load balance is checked and called even more,this was extremely messy because the checks and the calls to load balance were all mingled. It was a freaking mess.

[blaisb]

I also updated a test that was behaving very weirdly (aka it was supposed to load balance every 200000k iterations, but would still do one load balancing even if the simulation had 100k iterations which makes no sense... This PR also fixes this odd behavior

[blaisb]

The revert to using std::function brings us a bit closer to the original implementation that used function pointers, although in a cleaner fashion. If you both (@OGaboriault and @voferreira ) think this is a cleaner implementation, then we can continue with it. However, in order to remain coherent, we need to do the same approach for the contact detection And the load balance. Now only the load balance is done with std::function. @OGaboriault can you modify the PR to use the same mechanism for the contact detection ?

[voferreira]

@blaisb To me it is a go. @OGaboriault Pin me and I will review it.

[blaisb]

I will rebase then merge after CI has passed :)