Add support for Loweheiser generator

[Georacer]

Summary

This PR adds support for Loweheiser-brand on-board generators.

Details

Simulation files for this generator already existed in ArduPilot (and some autotests). This PR adds the actual driver.

The bulk of the logic was written by @peterbarker, for ArduPilot 4.2. I did the rebasing to master.

Many thanks to Harris Aerial who sponsored this work.

I do not have hands-on experience with the hardware, so any and all reviews are welcome.

Testing

New autotests have been added to explore some of the functionality.

Harris Aerial have tested their rebased branch (along with other commits) and report that their aircraft carrying the generator works as before.

Loweheiser's people tested Harris' branch and report that their system works as expected.

Known issues

EFI - Generator interdependency

This driver has a common backend for the EFI and the generator, which belongs to AP_Generator. That means that the AP_EFI_Loweheiser define needs to take into account that the generator part is also defined. Otherwise, there are builds that include EFI, don't include Generator, and thus fail due to include dependency failures.
This seems to do the job:

#ifndef AP_EFI_LOWEHEISER_ENABLED
#define AP_EFI_LOWEHEISER_ENABLED AP_EFI_BACKEND_DEFAULT_ENABLED && AP_GENERATOR_LOWEHEISER_ENABLED
#endif

although AP_EFI now has a dependency on AP_Generator.

Prearm behaviour

Harris Aerial reports that if the generator is hot (from a previous run), and is set in "run mode", then the UA will arm even though the generator is not actually running.

Miscellaneous

The branch that @peterbarker worked on also contained some small improvements for the Intelligent Energy 2.4kW fuel cell. These were sneaked in this PR as well.

[Georacer]

The build fails seemingly due to .h include issues. I'm investigating.

[tridge]

&& HAL_PROGRAM_SIZE_LIMIT_KB > 1024

[tridge]

for custom build only use HAL_PROGRAM_SIZE_LIMIT_KB > 2048
that would be my preference

[Georacer]

Done.

[tridge]

@peterbarker I think you better review this one

[tridge]

needs to be added into Tools/scripts/build_options.py

[Hwurzburg]

This needs IFDEF additions to make it an optional addition(defaulted as not being included since its a rare application) as well as entries in build_options and extract_features

[Georacer]

needs to be added into Tools/scripts/build_options.py

Done. Please check my work, that's the first time I edit this file.

[Georacer]

This needs IFDEF additions to make it an optional addition(defaulted as not being included since its a rare application) as well as entries in build_options and extract_features

This has now been marked with HAL_PROGRAM_SIZE_LIMIT_KB > 2048, which means it will only get build in SITL by default (or at least on boards with woads of space).