connect response stages with instrument types

[crotwell]

It is often assumed that stage 1 is sensor, stage 2 is preamp, stage 3+ is logger, but this is just heuristic that can break. But does illustrate that it might be useful to actually say what equipment the stage belongs to.

Maybe add a new element to make this connection, something like:

<stage>
   <ofEquipment>SENSOR</ofEquipment>

where the text can be SENSOR, LOGGER, PREAMP. Are there other?

This would then allow responses without a preamp, for example to avoid the use of empty placeholder stages.

Note in Section 3.2.4 Stage 2: The Pre-Amplifier, it says:

Using place-holder pre-amps at stage 2 may not be good practice.