Adjust gps minimum noise for RTK usage

[RomanBapst]

Currently the gps noise parameters (EKF2_GPS_P_NOISE and EKF2_GPS_V_NOISE) set a limit for the minimum sensor measurement noise used for the calculation of the innovation variance.
When using RTK GPS the reported accuracy of the gps measurements will be significantly lower than the current values.
We need to investigate which minimum values make sense for the use of RTK.

Here's a plot showing innovations (red) / innovation variance (green) for local position X and reported GPS accuracy (blue) during a flight with RTK GPS.

[priseborough]

The default values have been reduced to take advantage of current generation HW, but it is not practical to use the RTK fix type or published accuracy to change these automatically, because there are some receivers that are very optimistic. Some advice in the Wiki on recommended values for good RTK receivers might help.