Revive substitutional mass transport

The dev/multispecies branch is now obsolete, and its functionalities need to be revived on the current version of the master. Rebasing it on the current master will be extremely difficult, as it was branched out of master in 2020 Dec. Functionalities of the branch include handling simulations with multiple species.

Tasks

• Integrate species concentrations into mesh initialization and mesh repair
• Integrate species concentrations into neighborhood generation and halo mesh exchange
• Modify material class functions to get forces and energy based on species concentrations
• Reverify lattice parameter vs concentration results for NiPd alloy
• Add a basic mass transport model