Initialize meta data via constructor or set defaults from saver

[casabre]

If a waveform is created from a greenfield and no data were loaded, the meta data are empty. In order to give some hints to the user what meta data are relevant, a pre-filled dictionary should be set with e.g. None values. Maybe the saver can provide the defaults.

Furthermore, it would be useful to use kwargs in order to populate the meta data dictionary while construction, e.g.

>>> wv = rsb_waveform.RsWaveform(clock=1e9)
>>> wav.meta[0].clock
1e9