Refactor background equilibrium into dedicated functions

[n-claes]

PR description

This PR deprecates the various equilibrium fields (rho_field, T_field, v_field and B_field) and favours dedicated functions to specify the background. A new background object is created with various instances containing pointers to the equilibrium functions, which can be subsequently called.

Note that this PR breaks the Legolas API in the user submodule. This change implies:

• No more global field variables in the equilibrium module. These were imported were needed and it was hard to keep track of them.
• Reduced cross-module dependencies. The background instance is initialised only once in the main program, and then passed on to the various subroutines. Dedicated intent(in) statements prevent unwanted changes in the background state.
• The equilibrium functions are position dependent, such that they can be evaluated for any x. This means that we no longer have to interpolate between the base grid, Gaussian grid or eigenfunction grid (eg. for derived eigenfunctions).
• More user-friendly at the expense of slightly more code, but everything should be more clear. Users no longer need to take the grid into account when setting the background.
• Less memory usage, especially at high resolutions. The new functions eliminate the need to store the background in arrays and can be evaluated on-the-fly.

Before

real(dp) :: x
integer :: i 
do i = 1, size(grid_gauss)
    rho_field%rho0(i) = 2.0d0 * x**2
    rho_field%d_rho0_dr(i) = 4.0d0 * x
end do

After this PR

call background%set_density_funcs(rho0, drho0)

real(dp) function rho0(x)
  real(dp), intent(in) :: x 
  rho0 = 2.0d0 * x**2
end function rho0

real(dp) function drho0(x)
  real(dp), intent(in) :: x 
  drho0 = 4.0d0 * x
end function drho0

New features

Legolas

• A new background object to govern the equilibrium background state