rule to require both azimuth and dip or neither

[crotwell]

See FDSN/StationXML#154

Consider rule to require either both azimuth and dip on channels or neither. Current rule 320 requires seismic channels to have both, but preventing other channels from having just dip or just azimuth would be good.

Possible exception for vertical channels, dip exactly -90 or 90, where only dip is really needed.