Filter: LowPass: use calc_lowpass_alpha_dt in apply and prefix all class varables with _

[IamPete1]

A small simplification to our base low pass filter. It now uses calc_lowpass_alpha_dt and apply so the existing code is reused rather than being re-implemented.

There are some changes in behavior in the invalid dt/cutoff_freq cases. Previously the local copy of alpha was not updated where as it now is. This should not matter because the two apply methods should not be used together.

Future work
I would like to split the filter in to two classes one for each of the methods outlined here:

ardupilot/libraries/Filter/LowPassFilter.h

Lines 25 to 39 in 5a54d9a

	1) providing dt on every sample, and calling apply like this:
	// call once
	filter.set_cutoff_frequency(frequency_hz);
	// then on each sample
	output = filter.apply(sample, dt);
	2) providing a sample freq and cutoff_freq once at start
	// call once
	filter.set_cutoff_frequency(sample_freq, frequency_hz);
	// then on each sample
	output = filter.apply(sample);

The two methods should not be mixed, so separate classes will ensure that.

Then I would like to add a filter class which includes the cutoff frequency parameter since that is a very common usecase.

[IamPete1]

note that we are already using the calc_lowpass_alpha_dt function in this case.

[andyp1per]

should this constrain be added to calc_lowpass_alpha_dt?

[IamPete1]

See: #20911

The constrain will never trigger because we have already checked both values are > 0.

[peterbarker]

I thought we were trying to canonicalise the other way, to remove leading underscores over time

[tridge]

it would be good to be sure the two API approaches are not currently mixed

[IamPete1]

replaced by #27932