fix(apu): Added xfeed APU capabilities

[Taz5150]

Fixes #[issue_no]

Summary of Changes

This PR fixes/ adds cross feed APU fuel capabilities.

With xfeed OFF, APU will always burn fuel from the left tank.
With xfeed ON, APU burns fuel from both tanks as per the table below

Screenshots (if necessary)

References

Additional context

Discord username (if different from GitHub): TazX [Z+1]#405

Testing instructions

Starting with the aircraft C&D:

1. Proceed to start the APU
2. Press Xfeed PB to ON
3. Check fuel is burnt from both Left & Right tanks
4. Turn both Left fuel pumps OFF; check fuel is burnt from Right tank
5. Turn both Left pumps ON
6. Turn both Right pumps OFF; check fuel is burnt from Left tank
7. Turn all fuel pumps OFF; check fuel is burnt from both Left & Right tanks

How to download the PR for QA

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1. Make sure you are signed in to GitHub
2. Click on the Checks tab on the PR
3. On the left side, click on the bottom PR tab
4. Click on the A32NX download link at the bottom of the page

[tracernz]

Why make the text file invalid by removing the last line feed? :P

[donstim]

LGTM

[2hwk]

Needs modification to account for fuel burn when left and right pumps are off (see discussion in #development)

[Taz5150]

Needs modification to account for fuel burn when left and right pumps are off (see discussion in #development)

After some discussion in #development, I have updated the code as per the following table:

[BlueberryKing]

Dev Team Report

Discord : BlueberryKing
Object of testing: 8216
Tier of Testing : 1
Date : 14/10/2023

Testing Process:
I checked some fuel flow values by making sure the values on the SD page went down, but later just looked at the simvar values because the process was very slow.

• Start C&D
• Proceed to start the APU
• Press Xfeed PB to ON
• Checked fuel is burnt from both Left & Right tanks
• Turned both Left fuel pumps OFF; checked that fuel is burnt from Right tank
• Turned both Left pumps ON
• Turned both Right pumps OFF; checked that fuel is burnt from Left tank
• Turned all fuel pumps OFF; checked fuel is burnt from both Left & Right tanks

Testing Results:
Passed

Media:
N/A

Conclusion:
LGTM!