Defer creation of HDF5 file until prepare/trigger

[callumforrester]

Calling in prepare/trigger rather than stage would mean we can tie the file path into contextual information about the run. stage is normally called before open_run so creating the file in stage means the run does not exist at the time of creation. Any solution has to be mindful of pre-declare, the file must exist by the time describe is called, i.e. the declare_stream plan.

[abbiemery]

This is needed so we can use multipliers. Closing old issue #129 in favour of this.

[coretl]

Suggested changes to StandardDetector:

• As per #130 TriggerInfo should add:

  #: How many triggers make up a single StreamDatum index, to allow multiple frames
  #: from a faster detector to be zipped with a single frame from a slow detector
  #: e.g. if num=10 and multiplier=5 then the detector will take 10 frames, but publish
  #: 2 indices, and describe() will show a shape of (5, h, w)
  multiplier: int = 1

• stage() should only call writer.close(), not writer.open(), and should set self._trigger_info to None
• _prepare() will set multipliers based on the TriggerInfo, then call writer.open() with it
• As per #123, only set self._trigger_info if it has not already been set by prepare(), and check that self._trigger_info.trigger==TriggerInfo.internal

Should be done with knowledge of how the interface between StandardDetector and DetectorController will change in #405