Forced Mode feature addition for #2

[Gadgetoid]

This adds the ability to run the bmp280 in "forced" mode, which requests a single conversion from the sensor after which it returns to sleep mode.

If you're using a large delay between temperature readings in your code, this should help to eliminate any temperature offset caused by the internal temperature of the sensor itself.

The setup method now also accepts temperature and pressure oversampling and temperature standby (the time between measurements in normal mode) settings.

[coveralls]

Pull Request Test Coverage Report for Build 25

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+3.6%) to 97.938%

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Files with Coverage Reduction	New Missed Lines	%
bmp280/init.py	2	97.94%

Totals
Change from base Build 22:	3.6%
Covered Lines:	95
Relevant Lines:	97

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💛 - Coveralls