v0.16.0

Added

• WriteXML(elem any, w io.Writer, indent string, withHeader bool) exported
  function that streams the XML encoding of any element to w. (*MPD).Write
  now delegates to it.
• (*Period).Write(w io.Writer, indent string) and
  (*Period).WriteToString(indent string) for serializing a single Period.
• SCTE-35 splice information signalling on EventType per ANSI/SCTE 35
  2023r2: new Signal (SCTE 214-1 wrapper) and SpliceInfoSection
  (direct-under-Event) fields, all six splice commands and six splice
  descriptors, plus Binary, Ext and anyAttribute extensibility.
• SCTE-35 namespace pass-through: the URI seen on unmarshal
  (http://www.scte.org/schemas/35 or the legacy …/35/2016) is replayed
  verbatim on marshal.