Check validity of instances

[leonlan]

Except for some edge weight format and types, the current code does not check if the instance data are correct. For example, time windows can be checked for early <= late. Or that all data sections should have enough entries.

As of now, the library is just really about reading existing instances, so there should be no need to checking validity of the instances. But at some point it might be useful to include this as well, combined with #34 and #14.

[leonlan]

We should be slightly careful with this, because it depends on the specific problem formulation to define whether an instance is valid or not.

[leonlan]

I have decided that this and #27 is not something we will support. Our library focuses on input/output of VRPLIB instances. Validating the instance content is outside the scope of this library.