Steffen Brand

As DMN 1.1 supports multiple decision tables per file, this library should support this as well.

The methods validateInputs and validateOutputs are almost identical. And so are validateInputEntries and validateOutputEntries. https://scrutinizer-ci.com/g/steffenbrand/dmn-decision-tables/inspections/664a179b-e727-4d5d-bbae-9e36de69b32f/code-structure/operation/SteffenBrand%5CDmnDecisionTables%5CValidator%5CDecisionTableValidator%3A%3AvalidateInputs

Currently the whole XML string is build via string concatenation in toDMN method from DmnConvertibleInterface. Better use a library like [sabre/xml](https://packagist.org/packages/sabre/xml)

Move validation from DecisionTableBuilder build method to DecisionTable constructor, because that seems to be builder patterns best practice.