Add constants for gas heat enable/disable

[Gadgetoid]

Since the logic is reversed and setting the heat_off bit to 1 disables the heater, these constants should avoid confusion.

[coveralls]

Pull Request Test Coverage Report for Build 831168141

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.06%) to 84.431%

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Files with Coverage Reduction	New Missed Lines	%
bme680/constants.py	1	99.62%

Totals
Change from base Build 830992633:	0.06%
Covered Lines:	423
Relevant Lines:	501

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💛 - Coveralls