User interface for adding curves and plots - consolidation of plotting methods

fig.plot(...) adds a curve on the first plotwidget. Multiple calls of fig.plot(...) add multiple curves. In contrast, fig.add_plot(...)`, adds a new plotwidget + curve in the figure.

If we now like to add a 2nd curve to the 2nd plot, the access pattern changes. -->

ax = fig.axes[1]
ax.add_curve_scan(...)

This is confusing.Expected Syntax:

• fig.plot(...) adds a curve to the first plot
• ax = fig.add_plot(...) adds a new plot + curve
• With ax being an axis, the more transparent syntax would be ax.plot() to add a new curve instead of ax.add_curve_scan or ax.add_curve_custom. Give that .plot implements both (custom_curve and curve_scan)