lib: Simplify GLL summation mode detection.

[ossama-othman]

Change GLL optical axis type from double to size_t to simplify the summation mode detection logic by removing the need to be concerned about comparisons of numbers that cannot be exactly represented as floating point values.

[coveralls]

Pull Request Test Coverage Report for Build 2982894617

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.099%

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Totals
Change from base Build 2980695305:	0.0%
Covered Lines:	3185
Relevant Lines:	3535

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💛 - Coveralls