Improve plotting and add pulse_mode

[BoxiLi]

Description

• The plotting method is made more robust.
• Add pulse_mode as a wrapper for spline_kind, for discrete and continuous pulses. They are more friendly names for experimentalists.
• Add num_qubits as a replacement for N. In QuTiP capital letter are used for operators, like H for Hamiltonian. Due to historical reason, N is used in qip for the number of qubits. We would like to replace it with num_qubits in qutip_qip. Start from this wrapper and promoting the usage of this in notebooks guide.

Changelog
Minor

[coveralls]

Coverage decreased (-0.08%) to 63.125% when pulling c0a565c on BoxiLi:improve_plotting_and_add_pulse_mode into c1e05a7 on qutip:master.

[jakelishman]

Could remain outside the loop, but probably just personal preference:

for i, label_group in ...:
    [plotting stuff]
if axis and title:
    axis[0].set_title(title)

Is there a reason it only sets the title for one axis? I'm sure there is, just I don't see it immediately.

[BoxiLi]

Indeed this is on purpose, n axes are arranged in a column and they need only one title. It can also be down with fig I guess but I think set the title for axis[0] is safer for the position of the title.

[jakelishman]

LGTM

[BoxiLi]

There is a failing test for mesolve coeff, which I don't think is relevant to this PR.
And another test failing in test_zheevr.

[jakelishman]

I never saw the test failure in test_zheevr. That's odd - I thought we'd merged a change that fixed that. I certainly haven't seen one for a very long time personally - it's all test_diag_liou_mult.