Bootstrap sampling

[kuchaale]

Bootstrap sampling (see kuchaale/grl_2020/code/montecarlo_composites_script-faster.py) where we could consider error estimates _err in the sampling.

[kuchaale]

@gast-ben, can you open pull request here so that I am able to continue with your code?

[gast-ben]

Pull request was created in the new branch 'bootstrap' #26