Handle rounding conventions

[leonlan]

PyVRP uses the DIMACS rounding convention for two of its benchmark sets. This rounding convention can be added to the VRPLIB format so that we don't have to pass a custom round_func to each benchmark instance set.

[N-Wouda]

As a specification? I think that'd make sense. We can just support the values we already support in PyVRP's read function, and simplify read a bit (because the instance_format and round_func arguments would both become superfluous).