FramedArrays with uneven cell sizes

I have added FramedArrays with uneven cell sizes. The call looks like this

```Python
p = biggles.FramedArray(2,1)
p.cellspacing =	0.0
p.row_fractions = [0.8,0.2]

x = np.arange(100)
y1 = x*1.0
y2 = np.random.normal(size=len(x))
y1 += y2

p[0,0].add(biggles.Curve(x,y1))
p[0,0].add(biggles.Curve(x,y1-y2))
p[1,0].add(biggles.Points(x,y2))

p.show()
```