ardupilotmega.xml - remove empty parameters

[hamishwillee]

MAVLink docs/xml is unclear on the meaning of empty, reserved, and undefined parameters, and what the default values of each parameter should be. I've created this PR as a point of discussion before rolling out a consistent strategy.

TLDR;

• There is no safe way to override a param defined in a base XML, whether used or unused. Therefore all unused parameters marked as empty, reserved, or not defined mean the same thing, and should be marked consistently in XML.
• There are multiple ways of marking the default value of unused params, but these are ignored by consumers (they are set to 0).

Proposal:

• We delete all of "empty", reserved, etc params - if it isn't present it is assumed to be reserved. This is consistent, clearly means unused, and should reduce the amount of information to digest in XML.
• Default values are assumed to be 0 for all unused params, matching what the GCS do.

Whatever we end up agreeing I will roll out everywhere. FYI @auturgy @tridge @julianoes @peterbarker

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The long version:

Where we are now:

1. Inconsistent meaning naming and docs for unused parameters:

   • Kudos to ardupilotmega.xml, which only uses <param index="n">Empty</param>. Note though, it is not defined what this means if someone were to override the dialect.
   • common.xml has params that are marked as <param index="n">Empty</param> and <param index="n" reserved="true" />, and there are also some omitted params. It is not clear if these are the same or somehow different.
   • We added reserved as a mechanism in order to be able to do away with text and auto-generate it consistently if needed for docs. I.e. it was intended as a replacement.

2. Default values are defined to be settable in different ways but are ignored:

   • A default value is the value of the param that means "do nothing". It is important because if you have been sending 0 or NaN for the unimplemented param, you need to keep sending that value to mean "do nothing" - you can't send 0 and then decide later that the param is a temperature setting, and 0 means 0C (you have to say "0.1 means 0, and 0 means ignore).
   • COMMAND_INT and COMMAND_LONG specify that float params are default NaN and int params are default INT32_MAX
     • This can't work, because you don't know for sure which command a message will be sent in.
     • XSD allows you to specify a default value for a parameter such as <param index="n" reserved="true" default="NaN">, overriding the default.
     • Mission Planner and QGC tend to set 0 for all unused params. MAVSDK sets 0 for unused params on ArduPilot and NaN on PX4.

Proposal (duplicated from above):

• We delete all of "empty", reserved, etc params - if it isn't present it is assumed to be reserved.
• Default values are assumed to be 0 for all unused params, matching what the GCS do.

Reasoning for proposal above:

• W.r.t. deleting unused params vs having some standard format I am a little ambivalent. I do think though that if we have a standard format it should use the attribute "reserved" and auto-generate the docs - i.e. here replace <param index="n">Empty</param> with <param index="n" reserved="true" />. Important thing is to be consistent.
• I would love to specify that default values for unused params are: param1, 2, 3, 4, 7 = NaN, param5,6 to INT32_MAX. That should be easy - but no one is doing it so if we can't get people to buy into this, we should force 0 and have a simple rule
• I would love to override the default defaults using the default attribute :-) - but again, if no one is doing this, better to have a simple rule.