Refactor MPI utils

[fankiat]

Currently, we have separate files with separate classes for 2D and 3D MPI utils features. This can be generalized into dimension-agnostic code, so that users will not have to worry about the dimension of the MPIConstruct they are initializing, rather it will be inferred from the grid_size prescribed by the user.

I'm still not fully sure if we will benefit from this, and will see as we go when developing the 3D simulator. I'm putting this as low priority for now.

Things that need refactoring:

• Refactor MPI 3d utils to be consistent with MPI 2d utils #93
• Dimension-agnostic MPI utils