explain the algorithm a bit more

AP_OpticalFlow - small bug fix to ensure init attempts to read the product id 3 times before giving up

this caused problems with random()

This adds a large amount of noise robustness to the compass offset
learning algorithm, at a cost of 120 bytes of memory. The changes are
based on a long discussion with Bill Premerlani.

we don't need to add count any more, as floating point maths doesn't
need to round up

this seems to work much better than the old algorithm, converging
faster and more accurately. Even better, it has no linkage to DCM, so
no possibility of nasty feedback effects

APM_RC - moved Force_Out0_Out1, Force_Out2_Out3 and Force_Out6_Out6 to APM_RC parent class because it's already implemented in the APM1 and APM2 child classes anyway

Filter - added simple LowPassFilter (simple but it's possible to make errors with simple stuff too so might as well have one)

AP_OpticalFlow - resolved compile error in example sketch (it could not find DCM.h but it's not required anyway)

this is equivalent to multiplying by m.transposed(), but is more
efficient

this means we only link this in once, rather than for every use of
matrix multiply, which saves us some flash space

We need to be careful not to put large pieces of code in template
headers, as if the operation is used a lot, it costs us a lot of code
space

this implements a much more accurate model of a compass using matrix
rotations, instead of trying to calculate components directly

AC_PID - added more paranoid checking that imax is positive in constructor, operator() and load_gains methods

this allows us to use a tested and optimised rotation method

this is the core method used to update the DCM matrix with a gyro
vector. Moving it to AP_Math allows us to have a test for it

thanks to Justin for the suggestion

thanks to Justin for the suggestion

this will allow us to have an overall board rotation plus a per-sensor
rotation

this is the first step to creating a general AHRS class for ArduPilot

AP_Baro - removed unnecessary 2 element average filtering of pressure (there is a 4 or 5 element average filter in arducopter code itself, two places is messy)
        - also removed unused _offset_press variable

        - added average filter (for last two values) for raw pressure
        - changed some "long" to int32_t and "unsigned long" to uint32_t