clarify behaviour with respect to picks, station magnitudes, amplitudes

[kaestli]

The current specification does not include functionality to explicitly include picks, station magnitudes (and station magnitude contributions), or amplitudes into the response quakeml, or exclude it from there. Nor does it contain statements whether other request parameters should implicitly lead to the inclusion or exclusion of these event types. Thus users cannot be sure what behaviour to expect from specific fdsnws implementations.

• some web service implementations add picks (as far as referred to by amplitudes) to the response if the includearrivals=true option is set (and if they are available). This is reasonable, as the observed arrival time is part of the pick object -> i'd suggest to define this behaviour as the standard
• i'd suggest to add a new optional request parameter includestationmagnitudes=true|false, which should lead to the inclusion of station magnitudes, station magnitude contributions, amplitudes referred to by station magnitudes,, and picks (as far as referred to by amplitudes)

this could be done in a minor update of the specification, as it does not break compatibility or defined expectations.

Some data repositories and application cases include unassociated picks and unassociated amplitudes. While the current specification is not explicit on them, the context suggests that they are never returned. One may consider to allow for optional request parameters for returning them. These parameters should be mutually exclusive with the eventid parameter. start- and end time restrictions should also apply to these object types.