[PyBigDFT] Adjust band structure for supercells with non periodic directions.

Use a large size in case of non periodic direction, instead of 1. Like that ASE is not discretising any path along that direction. Like that, the graphene band structure is nice ;) Update also two calls to the ASE latest convention (at least the one in Ubuntu 20.04), to avoid warnings.

Also simplify a bit the code part to generate the evals(kpt) mesh for the interpolator.

@luigigenovese any remark ?

Before :

After :