Support for Online IO components

This MR, together with Moore!991 (merged), adds PyConf support for Online input and output components. Most notably the following things happen:

• Add make_raw_event_with_Online(), similar to make_data_with_FetchDataFromFile()
• Add online_writer(), similar to mdf_writer()
• default_raw_event() is restricted to only take exactly one bank type (i.e. one cannot request "the" raw event or a single raw event with two bank types) and Moore is adapted accordingly. This helps when adding the online input component (where there is no single raw event) but also is in line with the general direction of modernizing the raw event/bank data flow.
• Add exceptions for HltRoutingBits and HltDecReports banks in LHCb::RawEventCombiner.
• The functionality is tested in MooreOnline!69 (merged)

Goes with Moore!991 (merged) and MooreOnline!69 (merged)

Related to Online!528 (merged)