Refactor physics into dedicated functions

[n-claes]

PR description

Similar to #126 this PR deprecates the global physics fields (rc_field, eta_field, kappa_field, grav_field, hall_field) in favour of dedicated functions.

Note that this PR breaks the Legolas API in the user submodule. The same advantages as mentioned in #126 apply. Additionally, this allows users to override a specific physics pointer and provide a custom function instead. Function signatures are the same as for the background: position-dependent and returning a real(dp) value.

For example, setting gravity:
before

real(dp) :: x
integer :: i 
do i = 1, size(grid_gauss)
  grav_field%grav(i) = 1.0d0 / x**2
end do

After this PR

call physics%set_gravity_funcs(g0)

real(dp) function g0(x)
  real(dp), intent(in) :: x 
  g0 = 1.0d0 / x**2
end function g0

New features

Legolas

• A new physics object to govern the various physics functions.