plot broken scalar FE Types in broken plots

[pjaap]

This implements broken plots for broken scalar FE types.

Demo with dof-number as value for easy checking:

    xgrid = grid_unitsquare(Triangle2D)

    # broken FE space
    FES = FESpace{L2P0{1}}(xgrid)
    feVector = FEVector(FES)

    # value = dof for testing
    feVector.entries .= 1:(length(feVector.entries))

    ExtendableFEM.plot([grid(1),id(1)],feVector, Plotter = GLMakie)

and for L2P1:

with average_broken_plots = true we get the old behavior:

[pjaap]

If also works OOTB with slice plots (3D unitcube slices at x = 0.5):

[pjaap]

Removed the norm from the title in the plots. This was a C&P error.

[chmerdon]

Looks good!