require filter in ResponseStage

[crotwell]

Currently the filters (PolesZeros, etc) have minOccurs=0 on each within ResponseStageType. This allows a Stage like:

          <fsx:Stage number="2">
            <fsx:StageGain>
              <fsx:Value>1.000000000000E+00</fsx:Value>
              <fsx:Frequency>1.000000000000E+00</fsx:Frequency>
            </fsx:StageGain>
          </fsx:Stage>

to be valid. However, the comments imply that this should not be allowed:

There should be one response per stage.

In addition, this also means that the Stage does not have input and output units, which are important in understanding the overall response.

I believe the minOccurs attributes are leftover from an earlier version and should be removed, disallowing a Stage with only a StageGain.

For cases where the stage really is only a gain, there should still be a placeholder, like a unity Coefficients filter to provide units and so it is clear what is meant.

[crotwell]

Obviously this would be a breaking change, so would need to be for version 2.0

[crotwell]

Obviously this would be a breaking change, so would need to be for version 2.0.

A non-breaking change that would at least make it clearer what is going on would be to move the minOccurs to the choice like

<xs:choice minOccurs="0">