Add polymer background energy term

[mlund]

Add new energy term to implicitly describe the effect of grafted polymer chains to rigid molecules.

• Add new energy term inheriting from Energy::EnergyTerm.
  • Add declarations for src/energy.h and definitions to new src/energy_polymer.cpp. This will speed up compile times.
  • Take a list of molecular molecule names and their radii, e.g.:

  energy:
    - implicit_polymer:
        molecules: ["colloid1", "colloid2"]
        radii: [20.0, 30.0]
        polymer: {stiffness: 20.0, length: 10, molarity: 0.1}

  • Take polymer details (stiffness, length, ...)
• Add deterministic unit test of the energy term, e.g. using doctest in the .cpp file.
• Add test to examples/ folder. This could be a box of spherical colloids. JanF will likely have reference data we can compare against.
• Describe energy term in docs/_docs/energy.md
• Add energy term to docs/schema.yml