Default initialisers for properties and state in V4 XML

[m8pple]

In the v4 discussions we removed the ability to have default initialisers on property and state declarations in the xml:

https://github.com/POETSII/poets_improvement_proposals/blob/2a355282f3aa3912bdacf4ec50bf25199bdf310d/proposed/PIP-0020-v4-xml.md

Default initialisers are removed, and properties and state are zero-initialised if not given a value. This makes the parser simpler for any parsers that want to introspect data-structures.

At the time this was disallowed for a few reasons:

• Soton wanted things as simple as possible to get v4 working as soon as possible.
• Imperial was concerned about making the parser more complicated, as for non-Orchestrator tools it is necessary to fully parse the structure definitions.
• The ability to explicitly specify device state initialisers made it somewhat less important to be able to provide defaults.
• You can also set these things up in the OnInit handler if you want.

I see that the v4 versions of plate_heat use struct initialisers, for example:

https://github.com/POETSII/Orchestrator_examples/blob/f184e6d91aee06b3daaa96bba975f5f7e1ad6fb3/plate_heat/plate_heat_type_gals2.xml#L51-L56

This is actually not too big a deal if we want to support it. The parsing for properties/state structs turned out to be pretty easy, and I think defaults can be supported for those who need to completely parse declarations.

It's also a non-breaking change, as long as we bump the minorFormatVersion.

So if we want to support this in a v4.1 revision then that is quite doable.

[mvousden]

(tentative assignment)

[heliosfa]

It would be nice to support as I (personally) have found it makes things a bit easier. It is definitely not necessary, but a minor version bump would be acceptable.