Provide different scatter plots

Related to bob.measure#38 (closed)

We need scatter plots of:

• Multi-modal biometric systems

• Multi-modal PAD systems

• Bio and PAD

• with decision lines of a fusion system or threshold lines

Scatters with decision lines are already implemented however scatter plots like: Figure 7.a in http://publications.idiap.ch/downloads/papers/2017/Korshunov_STSP_2017.pdf are not implemented yet.