Add a skimmer base module

Addresses #8 (closed)

Fairly basic and straightforward skimmer module: the main interface method returns a selection and list of defined branches to save (instead of a list of plots for the histograms module).

The postprocess method may also write a modified analysis.yml file with the skimmed instead of the original samples, but we could also merge like this for now and figure that out later - please signal your urgency, for now I only tested

bambooRun -v -o test_skim_test -m bamboo.analysismodules:NanoAODSkimmerModule bamboo/examples/analysis_zmm2.yml --maxSelected=10

Note: this will only work on NanoAOD when using ROOT 6.16.00 (bug fix for branches with booleans), which can be picked up with

source /cvmfs/sft.cern.ch/lcg/views/LCG_95rc1python3/x86_64-centos7-gcc7-opt/setup.sh

I plan to make this the minimum required version as soon as the 'rc' is gone; with venv it's very easy to make a virtual environment based on that alongside the other one.