AP_STATS: allow writing to param STAT_RESET

[magicrub]

bugfix: param STAT_RESET should not be read-only. A fair chunk of code depends on it being able to change. Even the param description says (set to 0 to reset statistics)

[magicrub]

@amilcarlucas and do you remember the reason behind this? @peterbarker doesn't.

[khancyr]

this discussion will probably help : #6402

[rmackay9]

My guess is that MP's full parameter list will still allow you to set it to zero regardless of whether AP's param descriptions say it is read-only or not.

[amilcarlucas]

The point was that you should not mees up all the stats just because you loaded an old parameter file.

The statas are still resetable, but only if you explicitly write a zero to it.

[rmackay9]

@magicrub, let's close this one perhaps?

[magicrub]

At the very least we should update the docs:
Seconds since January 1st 2016 (Unix epoch+1451606400) since statistics reset (set to 0 to reset statistics)
to something like
Seconds since January 1st 2016 (Unix epoch+1451606400) since statistics reset. This is marked Read-only to inhibit accidental rights by popular GCS's. However, custom GCS's can set to 0 to reset the statistics

On another note:
The epoch offset of 1451606400 is only used by AP_STAT, the value of 1420070400 is used in 4 different places, including MAVLink and GPS. Doesn't mean we should change it.. just saying. Changing it would lead to breaking behavior on users expecting that number to not change.

[magicrub]

Dev call from Dec 25, 2023: @tridge says to re-word the description to not allow writes and new MAVLink msg to do the reset.