Build script for keopscore and pykeops

[gdurif]

Should fix #226

Clean generation of keopscore and pykeops distribution source packages (with the script pybuild.sh):

• Based on build and pyclean Python packages
• Everything is done in a local Python environment
• the keopscore version requirement is hard-coded in pykeops thanks to the generation of a setup.cfg file (that is cleaned afterward)
• A local build without hard-coded keopscore version requirement in pykeops can be done with the -l option
• pybuild.sh replaces pykeops/pykeops/generate_wheel.sh

./pybuild.sh -h
# Build script for keopscore/pykeops packages.
# 
# Usage: ./pybuild.sh [option...]
# 
#    -h     Print the help
#    -l     Build in local mode (without hard-coded keopscore version requirement in pykeops)
#    -v     Verbose mode
# 
# Note: by default, the keopscore version requirement is hard-coded in pykeops.

Remaining TODOs

• generate wheels using cibuildwheel (for another PR)