--union command-line argument for table-generator

[PhilippWendler]

As long as one has a list of result files that should be put next to each other in the table, one can call table-generator without an XML table definition, which is very convenient. However, if one was a list of result files that should be put below each other in the table (as with the <union> tag in a table definition) there is no convenient way to do this. While it is not meaningful to provide a command-line argument for every possible table structure, this use case seems easy to cover with a --union argument that makes table-generator behave as if all given result files were present in a <union> tag.

Example command lines:

table-generator --union foo.results.taskset1.xml.bz2 foo.results.taskset2.xml.bz2
table-generator -x table-definition-with-only-coluumns.xml --union foo.results.taskset1.xml.bz2 foo.results.taskset2.xml.bz2