Resolve "Optimize Maxwell energy calculation"

Description

Simplifies energy_mxll_calc considerably by using the energy_mxll_t object and implements the mask arrays for inner and boundary points to improve integration. Also, removes calls to energy_density_calc that are unnecessary. Now energy_mxll_calc calculates all the components of the energy.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

Closes #414 (closed)