Flint: Create custom curve stack

This MR introduce a way to browse a list of curve with a dedicated widget.

• The data have to be provided as a list of curves, or a 2D array
• The X-data can be provided (and is shared by the curves)
• A slider allow to browse the curves one by one

Designed for @berruyer

f = flint()

p = f.get_plot(plot_class="curvestack", name="curve-stack")

curves = numpy.empty((10, 100))
for i in range(10):
    curves[i] = numpy.sin(numpy.arange(100) / 30 + i * 6)
x = numpy.arange(100) * 10
p.set_data(curves=curves, x=x)