Enable integrating a function of fields

[jijn]

Currently we can only integrate a field, say u
$$\displaystyle \int_V u(x) ~dV$$

Here I provide function integrate_function to integrate any function of scalar fields
$$\displaystyle \int_V f(u_1(x),u_2(x),\cdots,u_n(x)) ~dV$$

For example, one field:
$$\displaystyle \int_V f(u(x)) ~dV$$

number I = Integrator<dim, degree, number>::integrate_function(
  solve_context.get_dof_manager().get_field_dof_handler(0),
  [](const number u_q) -> number {
    return std::exp(u_q);
  },
  solve_context.get_solution_indexer().get_solution_vector(0)
);

Two fields:
$$\displaystyle \int_V f(u(x),v(x)) ~dV$$

number I = Integrator<dim, degree, number>::integrate_function(
  dof_handler,
  [](const number u_q, const number v_q) -> number {
    return u_q * v_q;
  },
  solve_context.get_solution_indexer().get_solution_vector(0),
  solve_context.get_solution_indexer().get_solution_vector(1)
);

And more fields...

You can pass a scalar to the function, for example

number I = Integrator<dim, degree, number>::integrate_function(
  dof_handler,
  [V](const number mu_q) -> number {
    return 2.0*std::sinh(0.5*(mu_q + V));
  },
  solve_context.get_solution_indexer().get_solution_vector(0) // mu
);

You can define the function as lambda expression, traditional function, ....
We could, in principle, implement this function for any number of fields, but I think the current one should cover most use cases. This is done using variadic templates.
This PR prepares to implement galvanostatic conditions (Beck's approach).

[landinjm]

Any reason why the auto is here @fractalsbyx ?

[fractalsbyx]

no