Feature: catch streaming data

[tammojan]

We could make a mode in which DPPP reads not from a Measurement Set, but from a stream of data from the correlator. E.g. SPEAD. Of course this also requires some non-streaming communication about metadata is necessary, e.g. through a Telescope Management (TM) system. We could use https://gitlab.com/ska-telescope/cbf-sdp-emulator for simulating the correlator output, and a mock TM interface.