Support setting the units on the axes when plotting

[fhagemann]

using SolidStateDetectors, Unitful, Plots
sim = Simulation(SSD_examples[:InvertedCoax])
calculate_electric_potential!(sim)

Somehow, the current implementation (stable v0.9.0) does not allow to change the units of the axes easily, meaning that the following snippets of code kill the kernel:

plot(sim.electric_potential, xunit = u"mm", yunit = u"mm")

plot(u"mm", u"mm", u"V")
plot!(sim.electric_potential)

This should be fixed now by explicitly setting the default units to the internal units and by giving xlims and ylims units.

plot(sim.electric_potential, xunit = u"mm", yunit = u"mm")

works like a charm:

plot(u"mm", u"mm", u"V")
plot!(sim.electric_potential)

looks weird somehow (but people should then just not use it for now):

I updated the electric field plot in the tutorial, so that it's to be seen somewhere.

Other plots:

plot(sim.point_types, xunit = u"cm", yunit = u"cm", full_det = true)

plot(sim.q_eff_imp, xunit = u"cm", yunit = u"cm", zunit = u"MV * m", full_det = true)

[fhagemann]

This should be tested with all possible things there are to plot and with both coordinate systems, to be sure that nothing was messed up here.