Add validation program

[KrisThielemans]

yardl can currently not express constraints microsoft/yardl#176, but some might be added later. Nevertheless, it is unlikely that this will cover all constraints that would be useful to have. In addition, it would be suboptimal if a "reader" would check those constraints all the time, as this will likely reduce performance.

One way around this is to have a separate utility that "validates" a file: are all constraints satisfied? Is sufficient information present? This would be very useful to test output of converters.