Task #11013: reintegrate merge ('Debug subband mode for TBB_Writer'): fixes...

Task #11013: reintegrate merge ('Debug subband mode for TBB_Writer'): fixes TBB_Writer transient and spectral modes + adds 2 regression tests. Far from ready for operations, but ready for more (commissioning) testing. More development needed to solve known limitations. See also the 2nd part of the wiki page https://www.astron.nl/lofarwiki/doku.php?id=cep4:tbb