[Proof of Concept] Integrate energy added through "fixed" TLSPH particles

[efaulhaber]

This PR implements two versions of this.

1. Integration of energy with the main time integrator via an EnergyCalculatorSystem (introduced in Add energy calculator system #667).
2. A cheaper version that evaluates the forces on "fixed" particles only in a callback and then integrates the energy with the explicit Euler method. In my small experiment this added a 6% overhead vs 25% of version 1. It can be made even cheaper by applying the callback only every n steps at the cost of larger time integration errors.