Broadening options for weight computation

[raamana]

Using the new feature of arbitrary callable, implement more useful weights

• [ ] such as difference in medians,
• [ ] correlation, thresholding etc
• [ ] other of interest

[raamana]

03c5ebfc842864e0885e2746e1715aba2097f206 would pave the way for this.

Need to add another module (misc_metrics.py?) to achieve this.