Data Sources should allow "Optional" flag to prevent warnings in logs

[netniV]

Feature Request

Is your feature request related to a problem? Please describe

When you have a data source that may be optional, such as second or third phases on electrical circuits, without full separation of the graphs, you will get warnings about invalid responses.

Describe the solution you'd like

The ability to mark a data source as optional. This should ideally be at the template level to allow easy marking of optional sources.

<vbPhase1>
	<type>input-output</type>
	<oid>.1.3.1.2.1.3.45.1</oid>
	...
</vbPhase1>
<vbPhase2>
	<type>input-output</type>
	<oid>.1.3.1.2.1.3.45.2</oid>
	<optional>true</optional>
</vbPhase2>
<vbPhase3>
	<type>input-output</type>
	<oid>.1.3.1.2.1.3.45.3</oid>
	<optional>true</optional>
</vbPhase3>

Describe alternatives you've considered

Another alternative may be to have the optional as an attribute

<vbPhase2 optional>
</vbPhase2>

Consideration needs to take place for data input methods and potentially the Generic OID template.