Should support initialValue in AggregateArgumentType definitions

[merose]

It would be convenient if the initialValue specification in AggregateArgumentType definitions were used when sending a command or adding commands to a command stack.

[fqqb]

<AggregateArgumentType /> itself does not have initialValue support (no such field in XTCE, some other types do have it).
But I added a commit to #714 which should allow to specify what is requested.

Yamcs will then consider aggregate initial values in the following priority order:

1. initialValue attribute specified on the aggregate <Argument />.
2. initialValue attribute specfied on a specific member of the aggregate: <Member />
3. initialValue attribute specified on a the type of a specific member of the aggregate: <IntegerArgumentType />, etc

In case of (1), the aggregate value needs to be encoded in JSON (use " to escape ") format. Note that this may be Yamcs-only behavior, because XTCE does not appear to define how the initialValue attribute should be interpreted for aggregates (if at all).

In principle, the initialValue specified through (1) needs to be complete (that is: all members present). Although if a member is missing that does have initialValue set using (2) or (3), everything is merged together, and accepted by Yamcs.

By using (2) and (3) without (1), you could have some members of the aggregate initialized, and others not.

Here is an example for (1):

<ArgumentList>
	<Argument argumentTypeRef="Config_Item_Type"
                  name="CFGITEM" 
                  initialValue="{&quot;ADDRESS&quot;: 44, &quot;VALUE32&quot;: 6}" />
</ArgumentList>

[fqqb]

On master