Sensors: add parameters to calibrate the z-accel

This is redundant with the 3 parameters of the accel bias,
but by applying it as a separate term it avoids repeating
the six-point calibration on a regular basis.

This is useful especially for VTOL aircrafts where having a
good z calibration is especially critical for a good autonomous
hover.
Calibration: calculate the z-accel offset when leveling

This calculates the offset that will make the z-accel average
gravity after leveling.
Calibration: connect the bias leveling method to a button

The user can either click a button to calculate the board rotation with
or without zeroing the z-accelerometer.
Attitude: move the zero gyros option check box

It was grouped with the leveling button (historically) but is
an independent option so doesn't really belong there.
SetupWizard: use bias and leveling for MR

This method is better for multirotors as it results
in a better hover.
Merge pull request #1001 from peabody124/level_calibration

Level calibration
[Calibration]: This fixes a bad gyroscope calibration that would occur
when calibrating at unusual attitudes. Because the gyro bias accumulators
were working during the inital convergence phase, they would carry over
false data. This resolves the bug by forcing the integrators to 0 until
the calibration phase is completed.
[State estimation]: Refactor the accelerometer filtering pathway to make it
    clearer and shorter.
[Calibration]: Instead of resetting gyro bias, set bias error integral
parameter to 0.
[State estimation]: Fix bad attitude initialization.
[State estimation]: Miscellaneous improvements.