Update ledger/checkout/utils.py calculate_excl_gst

[wilsonc86]

set max digits precision on result to 22 and max decimal places to 12 with quantize

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 502

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.004%) to 39.749%

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Files with Coverage Reduction	New Missed Lines	%
parkstay/models.py	1	54.34%
ledger/payments/bpay/crn.py	1	55.22%
mooring/models.py	3	0.0%

Totals
Change from base Build 500:	0.004%
Covered Lines:	11456
Relevant Lines:	28821

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💛 - Coveralls

[scottp-dpaw]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved