Area element

[philippjfr]

Adds the new Area Element, which allows drawing the area under a curve:

xs = np.linspace(0, np.pi*4, 40)
hv.Area((xs, np.sin(xs)))

and between curves:

X  = np.linspace(0,3,200)
Y = X**2 + 3
Y2 = np.exp(X) + 2
Y3 = np.cos(X)
hv.Area((X, Y, Y2), vdims=['y', 'y2']) * hv.Area((X, Y, Y3), vdims=['y', 'y3'])

[jlstevens]

Looks good to merge as soon as the tests pass.

[jlstevens]

All the tests are passing now. Merging.