Refactor ReadBCParams

[lucafedeli88]

I would like to propose to refactor the ReadBCParams function (now defined in WarpXUtil), with the aim of:

• avoid manipulating directly the static variables of the WarpX class
• re-organize what the function does into smaller functions that only do one thing

Specifically, the changes are:

• create a new /FieldBoundaries.H/cpp file in BoundaryConditions, that for the moment contains only one pure function to parse the field boundary types from the inputfile (the idea is to extend this file later):

std::pair<
        amrex::Array<FieldBoundaryType, AMREX_SPACEDIM>,
        amrex::Array<FieldBoundaryType, AMREX_SPACEDIM>
    >
    warpx::boundary_conditions::parse_field_boundaries ()

• move the part of ReadBCParams that sets pp_geometry.addarr("is_periodic", geom_periodicity); according to boundary conditions to set_periodicity_according_to_boundary_types in Initialization/WarpXAMReXInit.cpp (we usually place here methods that change AMReX default parameters during the initialization).

• move the part of ReadBCParams that parses the particle boundary conditions to a new pure function parse_particle_boundaries in Particles/ParticleBoundaries.H/cpp (note that field boundary conditions are passed as arguments for consistency checks and for assigning default values):

    std::pair<
        amrex::Array<ParticleBoundaryType, AMREX_SPACEDIM>,
        amrex::Array<ParticleBoundaryType, AMREX_SPACEDIM>
    >
    warpx::particles::parse_particle_boundaries (
        const amrex::Array<FieldBoundaryType, AMREX_SPACEDIM>& field_boundary_lo,
        const amrex::Array<FieldBoundaryType, AMREX_SPACEDIM>& field_boundary_hi);

• initialize field_boundary_lo, field_boundary_hi, particle_boundary_lo, particle_boundary_hi inside WarpX::MakeWarpX

• Move consistency checks involving field solver to WarpX::ReadParameters (a further improvement would be to break ReadParameters into smaller pieces, but this will require several dedicated PRs).

• remove void ReadBCParams () from WarpXUtil.H/cpp