Refactoring EOS

Updates:

• refactored equation-of-state functions into templated classes;
• fixed evolution equation for internal energy;
• bugfix in several initial data generators

A few essential updates

Updated version contains:

• improved initial data generators;
• first-order fast multipole method (FMM);
• conservation of energy, momentum and angular momentum to machine precision;
• periodic boundary conditions;
• particle relaxation setup.

SodTube Release

Version with working example for SodTube and Fluid.