[plot] lineplot API same as matplotlib.pyplot.plot

Changing the new plot.line API to be similar to the API of matplotlib.pyplot.plot e.g.

line(mesh)  # z=const: y over x, y=const: z over x, x=const: z over y
line(mesh, ot.variables.temperature)    # temperature over x, y or z
line(mesh, "y", "temperature")          # temperature over y
line(mesh, ot.variables.pressure, "y")  # y over pressure
line(mesh, "pressure", "temperature")   # temperature over pressure

• Tests covering your feature were added?