Add possibility to set power mode, add function for calculate sea level pressure

[germsb]

Add setconfig function:

• set pressure oversampling
• set temperature oversampling
• set IIR filter
• set powermode
• set ODR (output data rate)
• set interrupt (data ready interrupt, see 3.7.5.3)

Add getSealevelPressure function:
Return sea level pressure in Pa according to the local altitude in meter and atmospheric pressure in Pa.

Modif performReading function to remove redundancies:
No need to redefine all settings at each reading, only power mode should be if it is set to forced mode.

Modif readAltitude function:
Sea level pressure param must be now defined in Pa. Now, for simplicity, all the params and values returned from the API are in Pa

Delete setTemperatureOversampling, setPressureOversampling, setIIRFilterCoeff and setOutputDataRate functions.
Not needed anymore.

It's my first coding in c/c++/ardui, I see this as an exercise, remarks are welcome :)
Have fun.

[ladyada]

hiya you removed some functions, which makes it impossible for us to merge. please try splitting the PR into multiple small branches for each change, but don't delete functions.

[germsb]

Indeed, I am merging the configuration functions into one.
The current implementation may seem correct, but it allows to exploit only a small part of the power of the sensor. Only the FORCED_MODE is supported which makes besides the setOutputDataRate function useless. There remains the possibility to configure the irr filter and temperature / pressure oversampling. Too bad when we see the possibilities of the sensor. Moreover, the implementation of the function performReading constrains to rewrite at each reading the totality of the configuration on the sensor whereas only the power mode must be it when defined on FORCED_MODE. In short, two writes for one read, it's not very efficient.
So yes, my pull request modifies how to use the library, but improves performance, provides the ability to set the power mode and therefore enjoy the outputdatarate setting in NORMAL_MODE, and also allows to use the sensor data ready interrupt function.
Before long, I plan to implement the fifo mode and correctly implement the different interrupt mode

[ladyada]

hiya its possible to do this withoutt breakint existing functionality - please update the PR and we can merge it