compute and display slope of speedup plots

[prudhomm]

we should compute the slopes of speedup using eg using linear regression, and display them in the legend next to the label

[tanguypierre]

Linear regression computed with sklearn.linear_model
Exact values, regression values and slopes are now displayed in a table after the graph, with a precision from 3 decimals

[tanguypierre]

Addition of an on/off button for displaying the regression line or not. (same color, but thinner and dashed line than real curve).
When activated, the dashed line appears in the legend showing the slope with a precision from 1 decimal.

Note: Complete interactivity couldn't have been reached
If a real curve is deselected, its regression line will appear by pushing the button