Add a flake for pybatsim

the flake only defines the pybatsim-core and pybatsim-functional packages. it also provides a nix template for the pybatsim example

in the flake of the template i commented out the input for the master pybatim sources/flake. If or when this commit will integrate master, we can remove the comments and thus provide a way to use the master version of pybatim and not the one in kapack