Setuptools fixes for development installation (faster rebuilds by caching the build dir)

source /cvmfs/sft.cern.ch/lcg/views/LCG_95apython3/x86_64-centos7-gcc8-opt/setup.sh
# cd to bamboo clone
mkdir devinstall ## if it does not already exist
export PYTHONPATH=$PYTHONPATH:$(pwd)/devinstall/lib/python3.6/site-packages
export PATH=$PATH:$(pwd)/devinstall/bin
python setup.py build
python setup.py install --prefix=$(pwd)/devinstall

creates a usable installation with these changes. Since the C++ libraries do not need to be rebuilt every time this shortens the turnaround time during development.

One warning: the build directory needs to be removed before installing from the same checkout with pip again.

Thanks @swertz for the suggestion and discussion!