Refactor boundary flags

[mrhardman]

This PR refactors the flags controlling boundary conditions, by adding

    impose_lower_boundary_condition::Bool=true,
    impose_upper_boundary_condition::Bool=true

permitting the user to include boundary points and also not impose a boundary condition there. This is useful in the case of cylindrical coordinates, where we may wish to define a coordinate on [0,L] and only impose boundary conditions at L. The previous implementation would require the use of points (0,L] to achieve the same outcome.

Use this feature on [0,L] by writing

radial = FiniteElementCoordinate("radial", ScalarCoordinateInputs(ngrid_radial,
                                nelement_radial,
                                0.0,Lradial,include_boundary_points),
                                weight_function=((r)-> 2.0*pi*r),
                                impose_lower_boundary_condition=false)

as compared to the option for (0,L] below.

 radial = FiniteElementCoordinate("radial", ScalarCoordinateInputs(ngrid_radial,
                                nelement_radial,
                                0.0,Lradial,exclude_lower_boundary_point),
                                weight_function=((r)-> 2.0*pi*r))