Add support for & in baseline regex

[gervandiepen]

With the current MSAntennaGram baseline selection it is virtually impossible to select LOFAR baselines between HBA0 and HBA1 in the same station. It could be achieved by adding support for selection on baseline name using &. For example a regex like:

(.*)HBA0&\1HBA1

will do the job. The & in the regex indicates it is special, so MSAntennaGram will form appropriate baseline names for all atenna pairs.