Add Validate method to RecordSet

[zacsketches]

See issue #30. Users should have the option to validate a RecordSet to ensure that the headers contain the right information in order to do follow on actions. This could take the form

func (rs *RecordSet) Validate(format FileFormat) (bool, error)

The FileFormat argument could be from a set of constants that define the list of file sources the package has been tested against. For example this set would contain MARINECADASTRE and INTERACTIONS.