AP_RPM: Giving RPM some TLC

[MattKear]

This is a minor fix to what I believe is just a hang over.

It looks like we create param arrays for _maximum, _minimum, and _quality_min yet we never give users the chance to set them on instances other than the 1st. This doesn't have any ploblems until we start trying to add more than two instances of RPM and immediatly follow on the param idx. Only noticed it whilst helping out @GiovannaAF trying to add more instances to RPM.

So we should either reduce the variable array size or add the _maximum, _minimum, and _quality_min params to the >1 instances of RPM IMHO. I opten for the former here:

[tridge]

as discussed on call, make 2_TYPE an enable param and add extra params for the 2nd instance

[MattKear]

Recommend reviewing per commit as there are a few things in this PR.

This PR now does the following:

• Fixes the bug whereby users cannot access _MIN and _Max params for RPM2 despite there being memory allocation for them.
• Gives each instance of RPM a _MIN_QUAL param.
• Puts RPM params behind an enable flag
• Re-factors the params in such a way so as to make it easier to expand the number of instances (I know of a few people wanting to do this)
• Removes type = PWM as this is a historic hangover from the PX4 split. I need to do the param conversion for the above point anyway so I figure this is the perfect time to do it.
• Minor whitespace tidy up.

[MattKear]

Wiki update here: ArduPilot/ardupilot_wiki#3701

[MattKear]

My apologies, I cannot attend the dev call but I think this PR is good to go. I would appriciate a review/potential merge.

[MattKear]

@Hwurzburg i already have a wiki pr to reflect these code changes.

[Hwurzburg]

I will make it delay merge until this gets merged...and you make the minor edits ...thanks

[MattKear]

Updated the param conversion note. Should be good to go once checks passed.

[IamPete1]

LGTM

Changes have been made